# Maths in IT #3: Algebra of sets

Hey everyone and welcome back! I’ve already announced this posts subject in my previous post, algebra of sets. So algebra is difficult and scary. At least that’s what people say (hmm hmm), so let’s shake that off. I’m sorry if you got the reference 🙂

If you haven’t read my previous posts I really suggest you do so as this post will continue where the previous one left off.

1. Maths in IT #1: Basic set theory
2. Maths in IT #2: Venn diagrams
3. Maths in IT #3: Algebra of sets
4. Coming soon…

Here’s a quick cheat sheet of symbols introduced in the previous post:
Intersection: $A \cap B = \{x | x \in A \land x \in B\}$
Union: $A \cup B = \{x | x \in A \lor x \in B\}$
Universe U: The collection of all elements we consider for our scenario.
Absolute complement (or simply complement): $A^c = \{x | x \in U \land x \notin A\}$
Relative complement of A in B: $B \cap A^c = A \setminus B$

## Algebra

What exactly is algebra? It’s about the study of symbols, or operations, and laws for manipulating these operations. Elementary algebra, for example, is a set of laws for operations (+, -, /, *) on numbers. Just like that $\cap, \cup$ and $A^c$ are operations on sets and the laws used to manipulating them is called the algebra of sets.

These laws can help us to simplify formulas. So let’s get started.

### Commutative property

Consider the following addition: 2 + 3. I’m pretty sure we all know the answer is 5 and the answer doesn’t change if we switch the numbers to 3 + 2. More abstract we can say that for every two numbers x and y goes that $x + y = y + x$.
When the order of operands (that is the objects on which an operation is performed) in an operation doesn’t matter for the outcome of that operation we call that operation commutative.

Multiplication (on real numbers) is commutative too, because $2 \cdot 3 = 3 \cdot 2$ (you may be used to seeing x or * as multiplication symbol, but you should remember $\cdot$ from high school). Minus isn’t commutative, because $3 - 2 \neq 2 - 3$ and neither is division, $2/3 \neq 3/2$.

When working with sets we can say that $A \cap B = B \cap A$ and $A \cup B = B \cup A$. That means both $\cap$ and $\cup$ are commutative.

### Associative property

When the order in which the same operation is performed in a single formula is unimportant for the outcome of that formula we say that the operation is associative. For example $(1 + 2) + 3 = 1 + (2 + 3)$. It doesn’t matter if we first evaluate 1 + 2 and then add 3 or first evaluate 2 + 3 and add it to 1, the outcome is always 6.
The same holds true for multiplication, because $(2 \cdot 3) \cdot 4 = 2 \cdot (3 \cdot 4)$ but not for subtraction, $(10 - 8) - 2 \neq 10 - (8 - 2)$ and division, $(16/4)/2 \neq 16/(4/2)$.

When looking at sets we find that both $\cap$ and $\cup$ are associative, because $(A \cap B) \cap C = A \cap (B \cap C)$ and $(A \cup B) \cup C = A \cup (B \cup C)$.

That means parenthesis can be omitted, removing any confusion on operator precedence.

And with the commutative and associative properties of both intersection and union we can now simplify
$C \cap (D \cap A) \cap B$
to $A \cap B \cap C \cap D$
or $C \cup (D \cup A) \cup B$
to $A \cup B \cup C \cup D$.
And when we combine intersection and union we can simplify as well. $D \cap (C \cup (B \cup A))$ can be simplified to $(A \cup B \cup C) \cap D$. Notice that we still need parenthesis to define the order of the $\cap$ and $\cup$ operations.

We can now also take the intersection or union of any number of sets because we don’t have to worry about the order of operands or operators. $A_1 \cap A_2 \cap A_3...$ gets tiresome really quick though. So suppose we have $A_1$ to $A_{100}$. The intersection or union of all those sets can now be expressed as follows:
$\displaystyle\bigcup_{i=1}^{100} A_i$     or     $\displaystyle\bigcap_{i=1}^{100} A_i$
And inline this look like $\cup_{i=1}^{100} A_i$ and $\cap_{i=1}^{100} A_i$.

Sometimes we don’t know if a set has 100 sets. Especially when a set is an infinite set of sets the above notation is not feasible. We can now use index notation to intersect or union all sets within the set.
$\displaystyle\bigcap_{i \in I} A_i$     or     $\displaystyle\bigcup_{i \in I} A_i$
$\cap_{i \in I} A_i$ and $\cup_{i \in I} A_i$ basically means “intersect/union for every set i in set I” (think of a foreach loop on a List of Lists in C#).

## Distributive property

The distributive property is difficult to describe in words. Just remember that multiplication is distributive over addition, meaning that $3 \cdot (4 + 5) = (3 \cdot 4) + (3 \cdot 5)$. More generally $x(y + z) = xy + xz$ (the multiplication symbol is often omitted between letters).

$\cap$ is distributive over $\cup$, meaning that
$A \cap (B \cup C) = (A \cap B) \cup (A \cap C)$

Likewise $\cup$ is distributive over $\cap$, so
$A \cup (B \cap C) = (A \cup B) \cap (A \cup C)$

We can now show that $(A \cap B) \cap (A \cup B) = A \cap B$. When we apply the distributive property on $(A \cap B) \cap (A \cup B)$ we get $((A \cap B) \cap A) \cup ((A \cap B) \cap B)$.
Here it is again, but with colors (and non-mathematical notation) so you can follow where everything went:
(A n B) n (A u B) = ((A n B) n A) u ((A n B) n B).
Applying commutativity and associativity we can now simplify further:
$((A \cap B) \cap A) \cup ((A \cap B) \cap B) = (A \cap A \cap B) \cup (A \cap B \cap B)$.
In the previous article, Maths in IT #2: Venn diagrams, we have seen that $A \cap A = A$ and $A \cup A = A$. This is called idempotence and can be applied here (twice) to simplify further:
$(A \cap A \cap B) \cup (A \cap B \cap B) = (A \cap B) \cup (A \cap B) = A \cap B$

Let’s repeat that:

$(A \cap B) \cap (A \cup B)$

Distributivity

$((A \cap B) \cap A) \cup ((A \cap B) \cap B)$

Associativity

$(A \cap B \cap A) \cup (A \cap B \cap B)$

Commutativity

$(A \cap A \cap B) \cup (A \cap B \cap B)$

Idempotence

$(A \cap B) \cup (A \cap B)$

Idempotence

$A \cap B$

So we could make use of distributivity, associativity, commutativity, and idempotence to make a rather difficult formula pretty easy! Unfortunately it was a lot less easy to do…

## Other properties

Unfortunately we’re not done yet. There are a few more properties, some of which we’ve already encountered, that I’d like to go over.

First we have the absorption law, which states that $A \cap (A \cup B) = A$ and $A \cup (A \cap B) = A$.

We’ve seen the so called null element, or empty set, $\emptyset$. Just like with numbers ($x + 0 = x$, $x - 0 = x$ and $x \cdot 0 = 0$) there are some rules for working with $\emptyset$. Luckily they’re not that hard.
$A \cap \emptyset = \emptyset$ and $A \cup \emptyset = A$.

When a universe U is defined we can say the following:
$A \cap U = A$ and $A \cup U = U$.

I’ve shown you that a double complement returns the original: $(A^c)^c = A$.
Also, $A \cap A^c = \emptyset$ and $A \cup A^c = U$.

## De Morgan’s Law

Finally we’ll look at the law of De Morgan, named after its inventor, the British mathematician Augustus De Morgan.
Suppose my blog can only be read by people who have not studied maths or IT. Let $A = \{x | \text{x has studied maths}\}$ and let $B = \{x | \text{x has studied IT}\}$. So now the people who may read my blog are people who haven’t studied maths or IT: $(A \cup B)^c$. I could also say people who haven’t studied maths and people who haven’t studied IT may read my blog: $A^c \cap B^c$. But that’s the same set of people!
In the following Venn diagram this is represented by the white part.

So in short $(A \cup B)^c = A^c \cap B^c$.

Now suppose my blog may be read only by people who haven’t studied both maths and IT. So that’s $(A \cap B)^c$. Or we could say people who haven’t studied maths and who haven’t studied IT, $A^c \cup B^c$. And again this is the same set of people. In the Venn diagram it’s the part where A and B don’t overlap (so almost everything).

And so that means $(A \cap B)^c = A^c \cup B^c$.

Now let’s take another example of how to simplify a formula. Suppose we have $B \cap (A \cap B)^c$. So let’s first apply De Morgan’s law: $B \cap (A \cap B)^c = B \cap (A^c \cup B^c)$. And now you may recognize a nice pattern for distributivity! So let’s apply distributivity, $B \cap (A^c \cup B^c) = (B \cap A^c) \cup (B \cap B^c)$. That’s nice, because we know that $B \cap B^c = \emptyset$. And now we know that $(B \cap A^c) \cup \emptyset = B \cap A^c$. And maybe you’ll remember this from my previous post, this is $B \setminus A$. And there you have it!

Again, let’s repeat:

$B \cap (A \cap B)^c$

De Morgan’s law

$B \cap (A^c \cup B^c)$

Distributivity

$(B \cap A^c) \cup (B \cap B^c)$

Complement rule

$(B \cap A^c) \cup \emptyset$

Null element

$B \cap A^c$

Relative complement

$B \setminus A$

And there you go! You algebra master, you!

So how will this help you in your day to day programming tasks? For most of us very little I’m afraid, except that you can work out some difficult set operations on paper and possibly simplify requirements. I’m no database developer, but I can imagine you’d need this when developing databases (and to a lesser extent when working with databases).

We did venture into the world of algebra though, and if you’re still with me I say congratulations! Algebra is a bit too abstract and difficult for many people.

I’m leaving you with a little cheat sheet of today’s lessons and I hope to see you again next time (no algebra, I promise)!

Commutativity:
$A \cap B = B \cap A$
$A \cup B = B \cup A$

Associativity:
$(A \cap B) \cap C = A \cap (B \cap C)$
$(A \cup B) \cup C = A \cup (B \cup C)$

Distributivity:
$A \cap (B \cup C) = (A \cap B) \cup (A \cap C)$
$A \cup (B \cap C) = (A \cup B) \cap (A \cup C)$

Idempotence:
$A \cup A = A$
$A \cap A = A$

Absorption:
$A \cup (A \cap B) = A$
$A \cap (A \cup B) = A$

Null element:
$A \cap \emptyset = \emptyset$
$A \cup \emptyset = A$

Universe:
$A \cap U = A$
$A \cup U = U$

Complement:
$A \cap A^c = \emptyset$
$A \cup A^c = U$

Double complement:
$(A^c)^c = A$

Relative complement:
$A \cap B^c = A \setminus B$

De Morgan’s Law:
$(A \cap B)^c = A^c \cup B^c$
$(A \cup B)^c = A^c \cap B^c$

# Maths in IT #2: Venn diagrams

Hey everyone, welcome back to part two of the Maths in IT series. I got a lot of positive response, so I guess I should just keep doing what I was already doing. This post will continue where part one left off, so if you haven’t read it I suggest you do so now before continuing.

1. Maths in IT #1: Basic set theory
2. Maths in IT #2: Venn diagrams
3. Maths in IT #3: Algebra of sets
4. Coming soon…

Here’s a quick cheat sheet with symbols I’ll use in this article:
Explicit definition: $A = \{a, b, c\}$
Implicit definition: $A = \{x | \text{ x is a letter in the alphabet}\}$
a is an element of A: $a \in A$
a is not an element of A: $a \notin A$
A is a subset of B: $A \subset B$
Empty set: $\emptyset$

## Venn diagrams

As promised we’ll use this post to combine sets. Before we do that let’s take a look at how to visually represent collection. We can do this using a Venn diagram. A Venn diagram is about as simple as it gets (although they can be pretty complex too). Each set is represented by a circle. The diagram can illustrate relationships between the represented sets.

Let’s look at an example. We have two collections, $A = \{a, b, c\}$ and $B = \{d, e, f\}$. Let’s show them in a Venn diagram.

When two sets have no shared elements, or for every $x \in A$ goes that $x \notin B$, we say the sets are disjoint.

Now suppose $A \subset B$ (A is a subset of B). We can show this in a Venn diagram and you’ll recognize it immediately.

When A is a subset of B then B overlaps A completely.

In the next sections we’ll combine sets and use Venn diagrams to visualize what elements we’re interested in.

## Intersection

So suppose we have two collections, $A = \{a, b, c, d\}$ and $B = \{c, d, e, f\}$. If I asked you which elements are in both A and B you’d answer c and d.

This is called the intersection of A and B.
We can write this as $A \cap B$.
In this example we can say $A \cap B = \{c, d\}$
More formally we say that $A \cap B = \{x | x \in A \text{ and } x \in B\}$.
And the symbol for “and” is actually $\land$, so to formalize it completely:

$A \cap B = \{x | x \in A \land x \in B\}$

Phew, that looks a whole lot like maths! You should read that as “the intersection of A and B is the collection of every x where x is an element of A and x is an element of B.”
And here is the Venn diagram, which visualizes this nicely. The intersection is the part in the thick black line where A and B overlap.

With an intersection we can give a formal definition of disjoint sets. When $A \cap B = \emptyset$ then A and B are disjoint.

Furthermore we can say that for any collection A goes that $A \cap \emptyset = \emptyset$.
Also $A \cap A = A$.
And when $A \subset B$ then $A \cap B = A$ (check the subset Venn diagram).

## Union

The intersection of two sets is the set of elements x where x is in A and B. Likewise, the union of two sets is the set of elements that are in set A or B. So when we have $A = \{a, b, c, d\}$ and $B = \{c, d, e, f\}$ then the union of A and B is $\{a, b, c, d, e, f\}$ (no doubles).

We can write a union of A and B as $A \cup B$.
Like $\land$ is the symbol for “and” $\lor$ is the symbol for “or”. So the formal definition of union is as follows:

$A \cup B = \{x | x \in A \lor x \in B\}$

Read that as “the union of A and B is the collection of every x where x is an element of A or x is an element of B.”
In a Venn diagram the union is basically just both sets (the part in the thick black line).

Unlike an intersection, a union of disjoint sets is not an empty set (notice that both sets have a thick black line).

Now for any collection A goes that $A \cup \emptyset = A$.
And, again, $A \cup A = A$.
Also when $A \subset B$ then $A \cup B = B$ (check the subset Venn diagram).

## Universe and Complement

When we’re talking about sets we’re usually not talking about that set in isolation. When I tell you that non-smokers live longer you understand that they live longer compared to people that do smoke. And you also understand that non-smokers and smokers combined make up for the worlds population. In this case we’re saying that we have a set of non-smokers in a universe of all people. We denote a universe with the capital letter U.

For every set $A_1, A_2, A_3... A_n$ goes that $A_n \subset U$.
That makes sense as U represents all elements we wish to consider for our case. No collection can ever contain an element that is not a part of all elements.

In a Venn diagram we draw a universe as a rectangle in which all our sets are drawn. In the following example N is the set of non-smokers and U is the universe containing all people.

Now with the notion of a universe we can say we want all elements that are not in any collection A. We call this the complement of A and we write it as $A^c$. In the following Venn diagram the white part represents $A^c$.

We can now formally define the complement:

$A^c = \{ x | x \in U \land x \notin A \}$

It goes without saying, but $\emptyset^c = U$ and $U^c = \emptyset$.
A little less obvious is that $(A^c)^c = A$. It makes sense though, as we first take everything that isn’t A (the complement of A) and then we take everything that isn’t in the resulting set, but that is A. Try drawing it in a Venn diagram and you’ll see what I mean.

A complement relative to a universe is called an absolute complement. A complement can also be relative to other sets. For example, when no universe is defined and we have sets A and B then the relative complement of A in B is the set of elements of B that are not in A. This takes the form of $B \cap A^c$ or $A \setminus B$.

$A \setminus B = \{ x | x \in B \land x \notin A \}$

In a Venn diagram $A \setminus B$ look as follows:

## Combining sets

We can now combine sets using intersection, union and complements. For example, let’s say our universe U is all living creatures on earth. Within U we have sets M, containing all mammals, B, containing all birds, and E, containing all animals that lay eggs. Formally:
$U = \{ x | \text{x is an animal} \}$
$M = \{ x | x \in U \land \text{x is a mammal} \}$
$B = \{ x | x \in U \land \text{x is a bird} \}$
$E = \{ x | x \in U \land \text{x lays eggs} \}$
Giving the following Venn diagram we can already draw some conclusions.

We can see that all birds lay eggs. Some mammals lay eggs too. No animal is both a bird and a mammal. As you see Venn diagrams can be really useful.
Now suppose we want the set of all mammals that lay eggs and also all birds. This is the collection $(M \cap E) \cup B$. That is, we take the intersection of M and E and union the result with B. In a Venn diagram we can see this collection (the red colored parts).

Now we can get every combination of sets using intersection, union and complement. It’s not always easy, but it’s possible.

## Some code

As promised I’m going to keep things practical. So what’s the practical use of all this? Well, most languages let you work with exactly these functions!

For example, take a look at this SQL expression.

(SELECT 1
UNION SELECT 2
UNION SELECT 3)
INTERSECT
(SELECT 3
UNION SELECT 4)

What will that return? It returns only 3 because 3 is an element of both collections. This example also shows the UNION operator. Basically this is the set $(\{1\} \cup \{2\} \cup \{3\}) \cap (\{3\} \cup \{4\})$.

And SQL knows complement too.

(SELECT 1
UNION SELECT 2
UNION SELECT 3)
EXCEPT
(SELECT 3
UNION SELECT 4)

What happens here is that we have (an implied) universe $U = \{1, 2, 3, 4\}$ and EXCEPT is the complement. So this query is basically the formula $\{3, 4\}^c$.
We could also say that U is not defined and this is the relative complement of $\{3, 4\}$ in $\{1, 2, 3\}$: $\{3, 4\} \setminus \{1, 2, 3\} = \{1, 2\}$.

But what about C#? In my previous post we’ve seen HashSet<T>, but I’m not going to use that now. Instead I’m just going with a List<T> as LINQ provides various extension methods for working with sets. Notice that HashSet<T> has its own methods in addition to the LINQ methods.

List<int> a = new List<int>();

List<int> b = new List<int>();

List<int> intersect = a.Intersect(b).ToList();
List<int> union = a.Union(b).ToList();
List<int> except = a.Except(b).ToList();

You may wonder what that looks like in Haskell (since I’ve shown you Haskell the last time too), but it’s not really that different.

Prelude> import Data.List
Prelude Data.List> [1, 2, 3] intersect [3, 4]
[3]
Prelude Data.List> [1, 2, 3] union [3, 4]
[1,2,3,4]
Prelude Data.List> [1, 2, 3] \\ [3, 4]
[1,2]

I’ve actually needed this stuff in my day to day work. It’s not that hard and sometimes it’s an explicit business requirement. For example I needed all sales orders from The Netherlands, Belgium and Luxembourg (the BeNeLux). That’s really just a union! And I’ve needed intersect too, give me all Dutch customers that are not in some list of customers. And how about all non-Dutch customers? That’s just the complement!

Next time I’d like to continue with algebra and algebra of sets in specific. Does that sound like it will give you nightmares? Don’t worry, I’ll be gentle!

See you next time!

# Maths in IT #1: Basic set theory

Welcome back everyone. Today I have a little something different for you. No new language or framework, but something that’s been around for millennia: maths.
When asking programmers about maths you’ll find two kinds of people, those who say you don’t need maths to be a good programmer and those who say maths is essential. Personally I think both are true. For some applications and industries you really don’t need advanced maths, but go into robotics, machine learning, statistics, or that kind of thing and you’re going to need maths, lots of it. And whether you need it or not, computers, programming languages and databases all wouldn’t exist without maths.
For now, let’s put it this way: knowing a thing or two about maths gives you an edge as a programmer!

Maths is everywhere: physics, chemistry, biology, economy and, yes, even in the arts! For this series I’ll focus on the maths we need in IT. I don’t know how much entries it’s going to have or what I’ll be discussing (and what I won’t be discussing), but I’ll be sure to keep it somewhat practical. No prior maths knowledge is assumed. Don’t worry, I’ll still post about code once in a while too!

1. Maths in IT #1: Basic set theory
2. Maths in IT #2: Venn diagrams
3. Maths in IT #3: Algebra of sets
4. Coming soon…

## Collections

Do I need to tell you why we should study collections? You probably use them in your code every day in the form of arrays, database tables, lists or hash tables. What you probably didn’t know is that collections have a lot of mathematical theory! Since collections are very important in both maths as in programming I’m going to start this series here. More specifically we’re going to talk about sets.

A set is an unordered collection containing only distinct values.

Let’s look at an example of a set in real life. Do you collect anything? Maybe you collect old records, each record in your collection can be uniquely identified and doubles are for trading or selling. Also, no matter in which order you put the records on the shelf, it’s still the same collection of records. So a record collection is really a set.

Likewise we can have a collection of paintings or stamps. Another collection is an alphabet, for example the English, Russian or Greek alphabet. Using these alphabets we can construct a language. We have natural languages (like the ones I just mentioned) and formal languages. Programming languages like C#, Java, C or Haskell are examples of formal languages.

A collection in maths is usually indicated by a single suggestive capital letter, such as $R$ for records, $S$ for stamps or $A$ for alphabet. If we have more than one collection we can use index notations to uniquely identify them: $A_{1}, A_{2}, A_{3}$

The notation for a single collection containing elements $a$, $b$ and $c$ is $\{a, b, c\}$. This is called the explicit definition. We can now declare a collection $A$ as $A = \{a, b, c\}$.
And of course we can have a collection of collections: $C = \{\{a, b\}, \{c, d\}, \{a, c\}\}$. Notice that collection $\{a, c\}$ is unique even though $a$ and $c$ are already elements in other collections.
The following set of sets isn’t valid: $\{\{a, b\}, \{b, a\}\}$. Because the order of elements in sets is ignored this set contains the same set twice, but sets also must have distinct values.
Now let’s say we have English alphabet $E$, Russian alphabet $R$ and Greek alphabet $G$. The collection of alphabets is written as $\{E, R, G\}$.

Now we want to indicate that $a$ is an elements of $E$. We do this using the following syntax: $a \in E$.
To indicate that $\lambda$ (the Greek letter lambda) is not an element of $E$ we use notation: $\lambda \notin E$.

We can compare collections. Collections are considered to be equal when they contain exactly the same elements, no more and no less. $\{a, b, c\} = \{a, b, c\}$, $\{a, b, c\} = \{c, b, a\}$ (remember, order is ignored), $\{a, b, c\} \neq \{d, e, f\}$ and $\{a, b, c\} \neq \{a, \{b\}, c\}$ (the collection $\{b\}$ does not equal $b$).

So far we’ve seen only explicitly defined collections. We can also implicitly define collections. This is especially useful when a collection contains too many elements to write down. For example a collection containing all countries on Earth. The notation for such a collection is as follows: $\{x | \text{x is a country on Earth}\}$. You should read that as “the collection consisting of all (objects) x for which x is a country on Earth”.
Generally we can say that an implicitly defined collection is in the form of $\{x | P(x)\}$ where, in this example, $P(x)$ is the statement that $x$ is a country on Earth. Actually $P(x)$ is a function taking parameter x and returning whether x is or isn’t a part of the set. We’ll look at functions in a later blog post.

To indicate how many elements are in any given (finite) collection we can use notation $|A|$. So $|\{a, b, c\}| = 3$ and $|E| = 26$ (where E is the English alphabet). Of course this is only possible when our collection is finite (we can count the elements).

If a collection is empty (it contains no elements) or some collection $A = \{\}$ we can use a special symbol $\emptyset$. And of course $|\emptyset| = 0$ (an empty set has $0$ elements).

When we allow the same element to appear in a collection more than once and we start taking the order of elements into consideration we’re speaking of a row. The syntax for a row is simply putting the elements next to each other. For example, using the set $\{a, b, c\}$ we can make the rows a, ababc, baac, caab, but not baad (because d is not in the set).

## Infinite collections

So far we’ve looked at finite collections.  Let’s look at some infinite collections now. Consider the collection of all numbers: 1, 2, 3, 4… In theory we can always add 1 to any number, so we can never stop counting. A few of these collections are so important that they got their own symbol.
First we have the natural numbers: $\mathbb{N} = \{0, 1, 2, 3, ...\}$.
If we add negatives to the collection we get all whole numbers, or integers: $\mathbb{Z} = \{..., -3, -2, -1, 0, 1, 2, 3, ...\}$.
If we also allow fractions, like $\frac{1}{2}$ (a half) or $\frac{1}{4}$ (a quarter) then we have the collection of rational numbers $\mathbb{Q}$.
Not all numbers can be expressed in fractions, for example pi ($\pi$, the surface of a circle with radius 1) or $\sqrt{2}$. When we want to include those numbers we get the collection of real numbers $\mathbb{R}$.

Now suppose we want all positive natural numbers, so 1, 2, 3… (excluding 0). We can indicate this with $\mathbb{N}^+$. Likewise, all negative numbers -1, -2, -3… can be indicated using $\mathbb{Z}^-$. And of course we can use $\mathbb{Q}^+$, $\mathbb{Q}^-$, $\mathbb{R}^+$ and $\mathbb{R}^-$ to indicate positive or negative fractions and real numbers too.

And we can now define new infinite collections using implicit definitions. The collection of all even numbers, for example, can be defined as follows: $E = \{ x | x \in \mathbb{Z} \text{ and x is even}\}$.
So here $E$ is a collection consisting of all x for which x is an element in $\mathbb{Z}$ (all integers) and x is even.

## Subsets

If a collection A contains elements that are all elements of another collection B we say that A is a subset of B. For example $A = \{a, b, c\}$ is a subset of $B = \{x | \text{x is a letter in the English alphabet}\}$ because a, b and c are all letters in the English alphabet.

More formally we can say that if A and B are both collections and for every $x \in A$ applies $x \in B$ then A is a subset of B.

We can write this as $A \subset B$. Since, in the previous example, A does not contain all letters in the English alphabet B we can also say that B is not a subset of A, this is written as $B \not\subset A$.

Given the above definition we can also say that $A \subset A$ or A is a subset of itself. The empty collection $\emptyset$ is a subset of all collections (including itself).

When a collection $A \subset B$, but $A \neq B$ then we say that A is a proper subset of B. We may write this as $A \nsubseteq B$.
Sometimes you may see the notation $A \subseteq B$ to indicate that A is a subset of B that may or may not be equal to B.
I’ll simply use $A \subset B$ throughout the series).

When $A \subset B$ and $B \subset A$ then $A = B$.

In a similar manner we can say that B is a superset of A if A is a subset of B. This is notated as $B \supset A$ (it’s the subset-symbol reversed). And of course we can say $B \supseteq A$ to indicate that B is a superset of A that may or may not be equal to A. For a proper superset we may use $A \nsupseteq B$ notation.

And when $A \supset B$ and $B \supset A$ then $A = B$.

## Some code

I promised I’d keep this series somewhat practical. So let’s look at some code. Consider the following C# sample using your favorite .NET collection class.

List<int> list = new List<int>();
int thirdItem = list[2]; // 0-based index.

The list now contains the items 1, 2, 3 and 1 again.  We can also get the item at the nth position, which is only useful when we know the order of the elements within the list. Based on that we can conclude that the List class in .NET is not a set. If we want a set in .NET we can use the HashSet class instead.

HashSet<int> set = new HashSet<int>();
{
}
//int secondItem = set[1]; // Doesn't compile.

Because a set only contains unique items a hash of each item can be generated which means that lookup time for sets is much faster than that of lists (especially when the size of the list grows). The HashSet<T> can be compared to the Dictionary<TKey, TValue> class, but without values.

Unfortunately C# doesn’t know list generators. Working with infinite collections isn’t very do-able in C# either. Let’s say you’d like to get all positive even numbers, $E^+ = \{ x | x \in \mathbb{N}^+ \text{ and x is even}\}$. We have a few problems. First $\mathbb{N}$ isn’t available in C#. We could take Int32.MaxValue (but that threw an OutOfMemoryException). So let’s just take all positive evens smaller than or equal to a million.

IEnumerable evens = from x in Enumerable.Range(1, 1000000)
where x % 2 == 0
select x;
List evaluatedEvens = evens.ToList();

Something like that is really the closest we can get in C# without going through a lot of trouble.

And at this point I stand corrected. Paulo Zemek pointed out that it’s actually pretty easy to work with infinite collections by utilizing the yield keyword. The next (Console Application) example illustrates this (although int will eventually overflow…).

static void Main(string[] args)
{
foreach (int i in Evens().Take(10))
{
Console.WriteLine(i);
}
}

static IEnumerable Evens()
{
return XToInfinity(1).Where(i => i % 2 == 0);
}

static IEnumerable XToInfinity(int start)
{
int current = start;
while (true)
{
yield return current;
current++;
}
}

That’s still a lot of typing though…

Let’s take another language that solves these kinds of problems a little better, a language that is a little closer to actual maths, Haskell.

evens = [ x | x <- [1..], x mod 2 == 0]

And there you have it. Notice that the code is actually pretty close to the mathematical notation. Take each x where x is an element of [1..] (one to infinity) and where x is even.
We can easily take the first 10 items of this infinite collection without crashing the program or encountering out of memory exceptions.

*Main> take 10 evens
[2,4,6,8,10,12,14,16,18,20]

This isn’t a blog on Haskell, so I can’t really expand on what it does, but I can say that Haskell is a “lazy” language, which means it doesn’t evaluate the items in the list until you actually need them. In this case it will only evaluate the first ten items of evens, keeping it from hogging our memory and CPU.

That’s it for now. We’ve had a very basic introduction to sets, but if you’re not familiar with this stuff it gets hard pretty quickly. So let’s take it nice and easy. Next time we’re going to combine collections and take a look at the Venn diagram.

Hope to see you next time!

# Apply, Call and Bind on JavaScript functions

I’m a bit short on time this week so I’ll keep it short. Last week I talked about Prototype in JavaScript, a fundamental JavaScript feature that too few programmers know about. Today I’d like to talk about a few more features that people either don’t know about or are confused about, apply(), call() and bind().

As usual I have the examples on GitHub in the apply-call-bind repository.

Also, in case you missed it, I have a free Udemy course on ReactJS and Flux for you. Only 50, so you best be quick to get it!

## Function invocation

So let’s talk a bit about functions and how they can be invoked. Let’s look at a simple function with a simple invocation.

var sum = function (a, b) {
return a + b;
};
var x = sum(1, 2);
console.log(x);

It doesn’t get simpler than this and x will have the expected value of 3. Let’s make that slightly less simple. We want to be able to add an undefined number of numbers! So how would we handle that? How about this?

var sum = function (numbers) {
var result = 0;
numbers.forEach(function(n) {
result += n;
});
return result;
};
var x = sum([1, 2, 3, 4]);
console.log(x);

That looks good, right? Unfortunately, invoking the function is now more difficult as we always need an array. What if I told you there is another way?

Remember that this is JavaScript and JavaScript is, well, weird. We can call a function with any number of input parameters we want. Too few and our function might fail, too much and the extra parameters will be ignored.
There is sense in all this though. Within each function you can inspect the input parameters through an array-like object, arguments.

Let’s rewrite the first version of sum, but using the arguments list this time.

var sum = function () {
return arguments[0] + arguments[1];
};
var x = sum(1, 2);
console.log(x);

But if we can do that we can use ANY number of parameters! Arguments is array-like, which mean we can loop through the values (it doesn’t have a forEach method though).

var sum = function () {
var result = 0;
var i = 0;
for (i; i < arguments.length; i++) {
result += arguments[i];
}
return result;
};
var x = sum(1, 2, 3, 4);
console.log(x);

That’s pretty nice! We can now use the same simple syntax for any number, or a variable number, of input parameters. So now I’m going to be a real pain in the ass… We got this nice function that takes any number of parameters in the plain and simple syntax we’re used to, but… I want to pass in an array!

## Apply

If I were your manager you’d be annoyed at best. I asked for plain and simple syntax, you deliver, and now I want an array after all!? Worry not! Your function is awesome and it can stay. What we’re going to use is a function on the function prototype (meaning all function objects have this function defined), apply.

So that sounded weird… Functions have functions? Yes they do! Remember that a function in JavaScript is just an object. To give you an idea, this is how it looks.

myFunc(); // Invocation of a function.
myFunc.someFunc(); // Invocation of a function on a function.

So let’s look at this function, apply, which allows you to pass input parameters to any function as an array. Apply has two input parameters, the object on which you want to invoke the function (or the object this points to in the function) and the array with input parameters to the function.

So let’s invoke our function, sum, using apply. Notice that the first parameter can be null as we’re not invoking sum on an object.

var x = sum.apply(null, [1, 2, 3, 4]);
console.log(x);

So that’s awesome, right? Let’s look at an example that doesn’t use the arguments variable.

var steve = {
firstName: 'Steve',
lastName: 'Ballmer',
doThatThing: function (what) {
console.log(this.firstName + ' ' + this.lastName
+ ': ' + what + '! ' + what + '! ' + what + '!');
}
};
steve.doThatThing.apply(null, ['Developers']);
steve.doThatThing.apply(steve, ['Developers']);

So in this example the function doThatThing is just a regular function with a regular named input parameter and we can still invoke it using apply. In this case we also need to pass in the first parameter to set this. If we don’t specify the first parameter firstName and lastName in the function will be undefined.

We can also put in another variable as first input parameter.

var sander = {
firstName: 'Sander',
lastName: 'Rossel'
};
steve.doThatThing.apply(sander, ['Blogs']);

That’s a bit weird, isn’t it? Even though Sander does not have the doThatThing function we can invoke it as if Sander did. This can be very useful behavior as we’ll see in the next section!

## Call

Another function, which looks like apply, is call. Call allows you to simply invoke a function, but makes you specify the object which serves as this within the function, much like apply. The only difference between apply and call is the manner in which input parameters are supplied. Apply needs an array with values, call needs the values separated by a comma (like regular function invocations).

steve.doThatThing.call(null, 'Developers');
steve.doThatThing.call(steve, 'Developers');
steve.doThatThing.call(sander, 'Blogs');

So why would you want to do this? It seems it only obfuscates the code… Consider the following scenario, you have some function that takes a callback as input and invokes it.

var someFunc = function (callback) {
console.log('this is ' + this);
callback('This');
};
someFunc(steve.doThatThing);

What will it print? Undefined undefined… Sure, when doThatThing is invoked as callback() this will be the global Window object. We have three options now… I’ll discuss two now and I’ll give the third in a minute.
So first we could make it so that this is not used in the callback. For this we’d need to create a closure, or a new function that invokes doThatThing on steve as if it were a normal function.

someFunc(function (what) {
steve.doThatThing(what);
});

It works, but its a lot of bloated code, so we don’t really want that. So there’s a second option, we allow the this object to be passed to the someFunc as an input parameter. And then we can invoke the function on that object using call (or apply)!

var someFuncThis = function (callback, thisArg) {
callback.call(thisArg, 'This');
};
someFuncThis(steve.doThatThing, steve);

A lot of JavaScript libraries and frameworks, like jQuery and Knockout.js, use this pattern, passing in this as an optional input parameter.

## Bind

If you’ve been paying attention you’ll remember I just said there was a third method to solve our little problem with this in the callback function. Next to apply and call functions also have a bind function. Bind is a function that takes an object as input parameter and returns another function. The function that is returned invokes the original function with the input object as this context. Let’s just look at an example.

someFunc(steve.doThatThing.bind(steve));
someFunc(steve.doThatThing.bind(sander));

And it even works if your original this isn’t the function itself.

var f = steve.doThatThing;
f = f.bind(steve);
someFunc(f);

You’ll need this when passing JavaScript native functions as callbacks to third party libraries.

someFunc(console.log);
someFunc(console.log.bind(console));

The first call fails with an “Illegal invocation” because this inside the log function has to be console, but it isn’t if it’s invoked as callback. The second line works as we’ve bound this to console.

So here are three functions that all invoke a function slightly differently than what you’re used to. All three are indispensable when you’re doing serious JavaScript development though.

If you’d like to know more about functions and how they work in JavaScript I can recommend JavaScript Succinctly.
We’ve also seen a lot of trouble with the this keyword. If you’re wondering why, you should check out this six day course by Derick Bailey: The Rules For Mastering JavaScript’s “this”. I can highly recommend it!

This blog is a bit shorter than usual, but, as they say, it’s quality over quantity. Hope to see you back again next week!

Happy coding!

# Prototype in JavaScript

Hey everyone. It’s been a few weeks since I last blogged. I’ve been on a vacation in Poland which was awesome! If you ever want to visit Poland I recommend visiting Gdańsk, a beautiful city, and Malbork Castle, the largest castle in the world!

So last time I finished my series on the MEAN stack, MongoDB, Express, AngularJS and Node.js. That meant a lot of JavaScript. Not just front-end, but also back-end. Next to my blog I’ve been writing a lot of JavaScript in my daily life too. And while writing all that JavaScript there was one thing I just didn’t quite understand, prototype. I’m not talking about the Prototype library, which kind of lost the battle for most popular all round JavaScript library to jQuery, I’m talking about prototypal object inheritance. I started asking around. I know some full-stack developers with years of experience in front- and back-end, but guess what? They didn’t fully understand prototype either. And then I went to look on the interwebs, but guess what? There just aren’t that many good prototype tutorials around. And that’s what this post is all about.

You can find the full code with examples on my GitHub in the prototype-blog repository.

## Prototype, you don’t really need it…

So when asking my friends why, after all these years, they still didn’t know prototype their answer was something along the lines of “I never needed it.” Sure, that was the reason I never looked into it before too. Truth is that you can write JavaScript apps and libraries without ever needing prototype. But if you want to write fast JavaScript code you better start using prototype.

So first of all, what is prototype and why is it better than no prototype? In JavaScript each function has a prototype property. Prototype has all the, I guess you could call it default, methods and properties that an object, created through that function using the ‘new’ keyword, should have. As we know any object in JavaScript can have any method or property, we can just define them at runtime as we go. And that’s probably the big difference between using prototype and not using it. With prototype you define methods design time, or up front.

Let’s look at an example. Let’s say I have a Person object, the constructor and usage could look as follows.

var Person = function (firstName, lastName) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
self.fullName = function () {
return self.firstName + ' ' + self.lastName;
};
};

var p = new Person('Sander', 'Rossel');
console.log(p.fullName());

So what happens when we call new Person(‘Sander’, ‘Rossel’)? First, two new instances of String are created, ‘Sander’ and ‘Rossel’. Second, a new object is created. After that the object, representing a Person, gets three new properties, firstName and lastName, which are assigned the two strings, and the function fullName, for which a new function is instantiated. That last part is crucial, a new function instance is created every time you create a new person object. Now performance and memory wise this isn’t optimal. Let’s see how we can optimize this.

var Person = function (firstName, lastName) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
};

var getFullName = function (person) {
return person.firstName + ' ' + person.lastName;
};

var p = new Person('Sander', 'Rossel');
console.log(getFullName(p));

This time the function getFullName is created once and can be used for any instance of Person. Don’t believe me? Let’s test that.

'use strict';

var benchmark = function(description, callback) {
var start = new Date().getTime();
for (var i = 0; i < 10000000; i++) {
callback();
}
console.log(description + ' took: ' + (new Date().getTime() - start));
};

var Person = function (firstName, lastName) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
self.fullName = function () {
return self.firstName + ' ' + self.lastName;
};
};

var PersonNoFullName = function (firstName, lastName) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
};

var getFullName = function (person) {
return person.firstName + ' ' + person.lastName;
};

benchmark('Full name', function () {
var p = new Person('Sander', 'Rossel');
var n = p.fullName();
});

benchmark('No full name', function () {
var p = new PersonNoFullName('Sander', 'Rossel');
var n = getFullName(p);
});

That’s a bit of code, but what happens is that we create 10.000.000 (that’s ten million) instances of Person with the fullName function defined on the Person object and we create ten million instances of a Person without the fullName function and use the pre-defined getFullName function instead. Then we log the time both methods took.

It may surprise you, but the result I get differs greatly per browser. Here they are (times in milliseconds):

                IE      FF      Chrome
Full name:      3705    361     2805
No full name:   3121    21      222

What this shows is that Firefox is fastest by far and IE is, of course, really very slow, especially when it comes to the method without full name. All browsers are considerably faster (especially Chrome) using the getFullName method though.

## Enter prototype

Now that we’ve seen that functions in your constructors aren’t really optimal, especially when they don’t use any internal state of an object, and that we’ve seen how to optimize that let’s look at a better way, prototype.

The use of an external function doesn’t feel right. Full name should be part of your Person object, but now we have to call some external function to get it. That’s where prototype comes in. Functions in JavaScript have a prototype property. Prototype defines static methods, like getFullName, and ‘pastes’ them on your objects through prototypal inheritance.

var PersonProto = function (firstName, lastName) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
};

PersonProto.prototype.fullName = function () {
return this.firstName + ' ' + this.lastName;
};

And that’s really all there is to it! We have now defined the fullName function on PersonProto’s prototype, which means every instance of PersonProto now gets the fullName function.

var p = new PersonProto('Sander', 'Rossel');
var n = p.fullName();

If we’d benchmark this we’d get about the same speed as we got earlier when using the getFullName function (really, try it).

You might be tempted to think it doesn’t matter whether you use a static method like getFullName or prototype, other than the manner in which you invoke the function (object.function() vs. function(object)). That isn’t true though, prototype has a few pros and cons compared to other methods. First of all, using prototype, we’ve lost control over ‘this’. Earlier, when defining fullName in the constructor we could use ‘self’, which always points to the correct instance of Person. We’ve lost that benefit with prototype and ‘this’ is now dependent on the context in which the function was invoked. Below code will break.

var p = new PersonProto('Sander', 'Rossel');
console.log(p.fullName());
var fn = p.fullName;
console.log(fn());

While the following code will work just fine.

var p = new Person('Sander', 'Rossel');
console.log(p.fullName());
var fn = p.fullName;
console.log(fn());

It’s the same code on the outside, but fullName of Person points to ‘self’ while fullName of PersonProto points to this (which, in the second call, is ‘window’).

Another pro, maybe, of prototype is that prototypal functions cannot be deleted from an object (although they can be overwritten). The first bit will run fine and prints ‘Sander Rossel’, the second part will break.

var p = new PersonProto('Sander', 'Rossel');
delete p.fullName;
console.log(p.fullName());

// This will break.
// fullName is no longer defined after a delete.
var p = new Person('Sander', 'Rossel');
delete p.fullName;
console.log(p.fullName());

And remember that prototype objects are static, which means they’re shared by all instances of an object. You can overwrite the value per instance though.

PersonProto.prototype.friends = [];
var p1 = new PersonProto('Sander', 'Rossel');
var p2 = new PersonProto('Bill', 'Gates');
p1.friends.push(p2);
console.log(p1.friends[0].fullName() + ' is a friend of ' + p1.fullName());
console.log(p2.friends[0].fullName() + ' is a friend of ' + p2.fullName());

Guess what, because the friends array is static Bill Gates is now also a friend of himself, even though I only added him to my own friends.

## Inheritance

One big pro to using prototype is inheritance. One object can inherit the prototype of another object. The ‘pseudoclassical’ way to do this is by copying the prototype object.

var Employee = function (firstName, lastName, salary) {
var self = this;
self.firstName = firstName;
self.lastName = lastName;
self.salary = salary;
};

Employee.prototype = PersonProto.prototype;

Employee.prototype.getSalary = function () {
return this.fullName() + ' earns ' + this.salary;
};

So we have an Employee with a salary and, of course, a name. An Employee is actually just a Person with a salary. So we ‘inherit’ the prototype of PersonProto. We also define a new function on the prototype, getSalary. Unfortunately, while Employee now has the fullName function, this has some undesired side effects…

var e = new Employee('Sander', 'Rossel', 1000000); // I wish :-)
console.log(e.getSalary());

var p = new PersonProto('Sander', 'Rossel');
console.log(p.getSalary());

That’s right, PersonProto now also has a getSalary function! That’s because Employee.prototype == PersonProto.prototype. So once we add getSalary to Employee.prototype PersonProto now has it too.

We can fix this by creating a temporary constructor, assigning the prototype to the temporary constructor, instantiating an object using the temporary constructor and assigning that to the prototype of the inheriting object. Last, because a prototype is assigned to the constructor function, we must set the prototype’s constructor to the inheriting constructor.

var Temp = function () {};
Temp.prototype = PersonProto.prototype;
Employee.prototype = new Temp();
Employee.prototype.constructor = Employee;

Employee.prototype.getSalary = function () {
return this.fullName() + ' earns ' + this.salary;
};

Alright, so I agree that’s pretty arcane! Luckily you can put this in a function so you only have to write it once.

var inherit = function (inheritor, inherited) {
var Temp = function () {};
Temp.prototype = inherited.prototype;
inheritor.prototype = new Temp();
inheritor.prototype.constructor = inheritor;
};
inherit(Employee, PersonProto);

And you can also see that Employee is actually also a PersonProto.

if (e instanceof Employee) {
console.log('e is an instance of Employee.');
}

if (e instanceof PersonProto) {
console.log('e is an instance of PersonProto.');
}

Another method to create an object with a specific prototype is by using the Object.create function.

var o = Object.create(PersonProto.prototype);
o.firstName = 'Sander';
o.lastName = 'Rossel';
console.log(o.fullName());

So here o has the PersonProto prototype and o is of the type PersonProto. Now let’s use Object.create for inheritance like we’ve seen above (for this example I’ve created an Employee2 which is similar to Employee).

Employee2.prototype = Object.create(PersonProto.prototype);
Employee2.prototype.constructor = Employee2;

Employee2.prototype.getSalary = function () {
return this.fullName() + ' earns ' + this.salary;
};

var e = new Employee2('Sander', 'Rossel', 1000000); // I wish :-)
console.log(e.getSalary());

And there you have it.

## Prototype vs. __proto__

As I said before prototype kind of ‘glues’ the static functions to the object instances you create. This happens through the proto property each object instance has. Each instance of an object has a proto property defined, which is constructed using the constructor.prototype. So while object literals don’t have a prototype property they do have a proto property which you can use to dynamically extend the prototype of an object.

var p = {
firstName: 'Sander',
lastName: 'Rossel'
};

if (p.__proto__ === Object.prototype) {
console.log('__proto__ points to the constructors prototype.');
}

// Highly discouraged!
p.__proto__.fullName = function () {
return this.firstName + ' ' + this.lastName;
};

console.log(p.fullName());
delete p.fullName;
console.log(p.fullName());

So by adding fullName to o.proto we’ve actually added fullName to Object.prototype! That means ALL your objects now have a fullName function.

var i = 10;
console.log(i.fullName());

See why doing that is highly discouraged? It’s slow to boot. I just thought you should know about it.

So that’s it about prototype. I have some final remarks for you before you go and use this knowledge out in the wild.
Don’t change the prototype of built-in types such as Object and Array. You could do this to support new functionality in older browsers, but there are probably a few libraries that already do that for you.
Prototype can have a negative impact on performance. What happened when we called fullName on Employee? The JavaScript engine looks for fullName on the object, but could not find it. It then looked for fullName on the Employee prototype, but again couldn’t find it. It works its way down the prototype chain and looked in the PersonProto prototype next, where it found fullName and called it. You can imagine that having huge prototype chains can be bad for performance (but why would you have huge prototype chains anyway?).
Last, as I said before, you can use JavaScript without ever needing prototype. Likewise, you can inherit objects without the need for prototype. For example, you can create a function Employee that creates a Person, adds the salary property and the getSalary function, and returns that.

And here’s some additional reading. First, of course, we have JavaScript Succinctly, which explain the various types, objects and functions, including prototype and inheritance.
And if you have some money to spare I can recommend one of the following books: Object-Oriented JavaScript by Packt Publishing or The Principles Of Object-Oriented JavaScript by No Starch Press.

Happy coding!

# MEAN web development #9: Some last remarks

So by now we’ve seen the full MEAN stack, MongoDB, Express, AngularJS and Node.js. Additionally we’ve seen the templating engine Jade in action as well as used Socket.io for real time web applications. If you combine that with my earlier series on ‘vanilla’ web development you’re already a pretty versatile web developer (well, dependent on how much you practiced)!
In case you missed a MEAN article, here they are:

In this post I’m going to discuss one topic we haven’t covered yet, deployment. Next to that I’m going to give you some alternatives for the technologies we’ve covered in this series.

There is no GitHub repository for this article as there won’t be any code samples.

## Deploying a Node.js application

So we haven’t talked about deployment yet. Not even in my web development series. To be honest, deploying software is not what I do, I just write them. Besides, deploying to the Windows laptop you’ve used to build your software is a bit difficult and in most cases isn’t even close to actual deployment. Unfortunately I don’t have any spare web servers laying around 🙂
That isn’t to say I can’t point you in the right direction.

Node.js is a little different from what you’re used to. Consider the PHP application we wrote earlier in Web development #4: PHP in the back. We wrote our page and got it up and running using XAMPP, where we specified the port and other settings. The PHP file didn’t do anything. Likewise, a C# web application doesn’t do anything until you host it using IIS, which can be configured however you like.
That’s different for a Node.js application. After all we specify the server, where it should run and how it should run in our JavaScript file. And then we could run it from the console. On production environments we really don’t want to run a console though. We probably want to host our application on port 80, which is the default HTTP port, which requires an elevated command prompt. So someone with admin rights should log on to the server to (re)start our Node.js app every time something happens (either the app crashes or the machine is restarted). That doesn’t sound very appealing…

So we could simply write some command script using a loop that restarts Node.js whenever it crashes and start that up, with elevated privileges, either using a service or the built-in scheduler. That solution has some serious limitations though. What if your app goes in an unrecoverable state, making it crash in a loop? Or what if, for some reason, it hogs up all of your memory? A command prompt can’t really give you a detailed log of what’s happening with your application.

That sounds really tiresome and messed up. Luckily there is a better alternative. You can use PM2 (Process Manager 2) to run Node.js (PM2 on GitHub). PM2 can run other scripts such as PHP, CoffeeScript and Ruby as well and it works on Linux, MacOSx and Windows.
You can install PM2 using npm (npm install pm2 -g). Make sure to add  the -g to make a global install. Now you can simply start any Node.js app by running “pm2 start node_app.js”. “pm2 list” will show a list with running applications. Then you can generate a startup script with “pm2 startup” to automatically start your apps on a computer reboot (which unfortunately doesn’t work on Windows, so I haven’t been able to test it). “pm2 monit” will show you some statistics on the current status of your applications.
Next to that PM2 has a deployment system, log management, a development tool (similar to nodemon which we’ve been using in this series), cluster mode (for running on multiple CPU’s, remember Node.js is single threaded) and much more which you can all use as well.

There are some alternatives to PM2, such as forever, but something completely different is to run your Node.js app as a Windows service. You can do this by using NSSM (the Non-Sucking Service Manager) and winser. You can download NSSM from their website and you can install winser using npm (npm install winser). Then you should add the following lines to your package.json:

"scripts": {
"postinstall": "winser -i -s -c",
"preuninstall": "winser -r -x -s",
}

Now in the command prompt browse to the folder that contains the package.json and execute “node_modules.bin\winser -i”. You have now installed your Node.js application as a service. To uninstall execute “node_modules.bin\winser -r”.

So I know that’s not much, but at least these are some considerations when deploying a Node.js application.

## HTTPS/SSL

You might want to deploy your application using HTTPS for secure SSL-encrypred traffic. For this you’re going to need some certificates, which I dont have. Node.js has an https module, just like it has a http module. So now it shouldn’t be too difficult to write yourself an HTTPS enabled Node.js server.

var https = require('https');
var fs = require('fs');

var options = {
};

var server = https.createServer(options, function (req, res) {
// Stuff...
});
server.listen(80, '127.0.0.1');

And here’s how you can use Express to handle HTTPS.

var app = require('express')();
var https = require('https');
var fs = require('fs');

var options = {
};

// Stuff...

var server = https.createServer(options, app);
server.listen(80, '127.0.0.1');

And now you also know why you should use packages for your routing, you may want to re-use them.

## MEAN alternatives

In this series we’ve seen the MEAN stack: MongoDB, Express, AngularJS and Node.js. It offers an alternative to the traditional LAMP stack, Linux, Apache, MySQL and PHP and the Microsoft stack with .NET and IIS. The baseline here is that we programmers just like to come up with cool or clever sounding acronyms. As I said before you can have a perfectly good MEN stack, leaving out the AngularJS. Or put in some Jade, or Sockets.io. Well, people have actually done that and came up with some other nice sounding acronyms. Here are some that I’ve seen: ANNE (AngularJS, Node.js, Neo4j, Express); BEANS (Bootstrap, Express, AngularJS, Node.js, Sockets.io); EARN (Express, AngularJS, Redis, Node.js). My point is that you’re not tied to MEAN, it’s simply a couple of technologies that work well together and can help you get things done.

If you like you can have a look at Sails.js, which is built on Express, but adds MVC and an ORM, among other things. Another alternative for Express (or Sails.js) is Hapi.js, which focuses more on configuration than code.

As alternative for MongoDB we’ve already seen Neo4j (a NoSQL graph database) and Redis (a NoSQL key-value store). They don’t actually have to be alternatives as you can use both in a single project. An actual alternative could be CouchDB, which is also a NoSQL document database and is probably the second most popular document database after MongoDB,
But who said you have to stick to NoSQL? If you’d like to use MySQL with Node.js that’s perfectly fine too!
If you just look at the Express database capabilities you’ll find databases such as Cassandra, PostgreSQL and ElasticSearch are supported out of the box.
And there are drivers for SQL Server and Oracle too.

Let’s look at some AngularJS alternatives. Of course there’s always jQuery that you can still use in your projects. Get some Knockout.js in your project too and you might not need AngularJS at all (although AngularJS is probably your first choice for SPAs (Single Page Applications).
While AngularJS is the most popular front end framework, it’s not your only choice. Underscore.js is another popular framework that is a little less bloated than AngularJS (but does less as well).
Another framework that you might want to try is Ember.js, which does pretty much everything that AngularJS does too. If you want to read more about Ember.js I can recommend Erik Hanchett’s blog.

And if you wish for another HTML template engine (or no template engine at all) you can ignore Jade and go with, for example, Mustache or Handlebars. Keep in mind you can use templating both on your back end and in your front end.

I’d mention alternatives for Node.js, but that would kind of defeat the purpose of MEAN and the (almost) all JavaScript stack. So let’s not do that (besides, you probably know them already).

## Other popular modules

Let’s quickly check out some other popular Node.js modules. I’ll let you figure out how to use them on your own, but I’ll give a quick overview and a link to the website (with documentation).

The first library you might want to check is Browserify, which let’s you use require() in the browser. That’s pretty sweet!

Another popular library that you can use in both Node.js and in your front end is async. It provides many functions for asynchronous execution, such as each, map and filter on arrays.
It can also help you with asynchronous flow control. That is executing one task after another in an elegant and asynchronous manner. And for this same purpose the creator of async also gives us Nimble. And you might want to take a look at the alternatives Seq and Step as well.

So how about your minification, compilation, unit testing, linting (checking for syntax errors) etc.? For these tasks you can use Gulp or Grunt. Both applications are popular ‘task runners’ and can be easily installed using npm (using -g) and automate these kinds of tasks. They might take some configuration and some time to get used to, but they’re worth it.

Last I’d like to point out commander. Since everything nowadays seems to be working with the command prompt you might want to make your application command prompt configurable too. Something like “node server.js -super” where -super (or -s for short) starts your application in superman mode. You don’t need a library for this, but it does come in handy. Commander seems to be the most popular one.

So that’s it for the entire MEAN series. I hope you’ve learned and enjoyed it as much as I have! This is the end of the MEAN series, but certainly not the end of my blogging career. In the future you may expect more posts on web development, JavaScript, NoSQL, and who knows what. I’m always open for suggestions!

For additional reading on MEAN and related technologies I can once again recommend the Succinctly series by Syncfusion. They have many free ebooks like Node.js Succinctly, AngularJS Succinctly and MongoDB Succinctly. I’m also a big fan of the books by Manning Publications. Though not free they’re definitely worth your money. Books like Node.js In Action (second edition coming up), AngularJS In Action, MongoDB In Action (also a second edition coming up) and even Express In Action. A book on the entire MEAN stack, Getting MEAN, is also in the writing (so get it early)!

I’ll be on a vacation in Poland for the next two weeks, so don’t expect a new blog post for at least another three weeks. Maybe a short one on a rainy day and if I have wi-fi in the hotel, but I’m not making any promises.

Thanks for sticking with me and happy coding!

# MEAN web development #8: Sockets will rock your socks!

This week I’m planning to keep it short. We’ll look at sockets and how to utilize them using Node.js. Luckily this is a relatively easy task for which Node.js is famous. Sockets enable you to build real time websites, something that is difficult at best using HTTP.
This will also be the last article in the series in which we’ll look at a new technology. If you read everything up til now you’re pretty well on your way to becoming a MEAN web developer! Next week we’ll have a wrap up and after that, who knows 🙂

As usual you can find the code samples for this post on my GitHub page under the mean8-blog repository.

## What are sockets?

I won’t really go into technical details about sockets, or networks sockets. What is important to know is that a socket is basically an endpoint connection that allows communication between computers in a network. In an earlier post, Web development #1: Internet and the World Wide Web, I have briefly discussed the internet protocols TCP/IP (Transmission Control Protocol/Internet Protocol) and I have mentioned other protocols such as UDP (User Datagram Protocol). Those protocols make use of sockets to transfer data across the web.

As we have seen protocols such as HTTP (and HTTPS) make use of the TCP/IP protocol, which makes use of sockets. As we know HTTP is a one way street, although the data flow isn’t. Think about it, with HTTP we can request data, but that data also has to be sent back to the client. What HTTP can’t do, but sockets obviously can, is sent data to a client. Now what if a server could send data to a client even though the client didn’t request it (at that exact moment)? Look no further! And that is exactly why sockets programming is becoming more and more popular!

The protocol used for this bi-directional exchange of data is called WebSockets and was standardized in 2011. As this is still pretty new keep in mind that older web browsers do not support this protocol. One thing to keep in mind is that WebSockets is really an upgrade from HTTP. A socket connection is actually established using an HTTP request.

Node.js has made it pretty easy to work with sockets. There are actually already quite a lot of tutorials out there and most of them let you build a chat application. Of course you can build anything requiring real time data. Think of real time Facebook or Twitter updates. It’s especially interesting when you consider the recent Internet of Things (or IoT) where everything is connected to the web. Let’s say someone rings your doorbell, which is connected to the web and sends you a picture of your front porch when someone rings the bell. Would you want to use HTTP to constantly poll/request if someone has just rang your doorbell? No you wouldn’t! In these scenario’s sockets are a necessity!

## Sockets and Node.js

So let’s set up a small Node.js server that supports sockets. We could use ‘pure’ WebSockets for this, but I’m going to use a library for this called socket.io. This will make our life a lot easier on both the back- and front-end. socket.io is a layer around WebSockets that simplifies the API and, more importantly, falls back on other methods if WebSockets isn’t supported.

We’ll start by creating a Node.js server that serves some HTML page. We’ve done that a few times before, so that should be no problem. Remember that WebSockets requires HTTP to work, so we’ll need the HTTP module as well. And, as always, I’ll be using Express as well.

var express = require('express');
var app = express();
var http = require('http').Server(app);

app.use(express.static('public'));

app.get(['/', '/index'], function (req, res) {
res.sendFile(__dirname + '/public/client.html');
});

http.listen(80, '127.0.0.1');

After that we can install socket.io (npm install socket.io).

var express = require('express');
var app = express();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.use(express.static('public'));

app.get(['/', '/index'], function (req, res) {
res.sendFile(__dirname + '/public/client.html');
});

io.on('connection', function(socket){
console.log('A user connected!');
});

http.listen(80, '127.0.0.1');

As you can see the http object is used to create an instance of socket.io. After that we can listen to the connection event for incoming sockets.

Now socket.io does a little magic trick, it automatically serves socket.io to our front-end. That means creating a socket instance on the client is really easy. First of all we need to include the script somewhere.

<html>
<meta charset="utf-8">
<title>Sockets example</title>
<script src="/socket.io/socket.io.js"></script>
<script src="client.js"></script>
<body>
<h1>Sockets example</h1>
</body>
</html>

And then we can use sockets in client.js.

var socket = io();

Wow, that was easy!

And you’ll notice that if you put that in /public/client.html and browse to localhost Node.js will log ‘A user connected!’ to the console! So we’re already connecting.

Now we’ve talked about EventEmitters before in MEAN web development #3: More Node.js. socket.io uses them extensively. Each socket has a special ”disconnect” event.

io.on('connection', function(socket){
console.log('A user connected!');
socket.on('disconnect', function () {
console.log('A user disconnected...');
});
});

If you try that, browse to localhost, and refresh the page, you’ll see “A user connectioned!”, “A user disconnected…” and “A user connected!” in the console. So this already works pretty sweet, right?

Let’s add another event. You know I like music and albums are always a favorite test object for me.

io.on('connection', function(socket){
console.log('A user connected!');
socket.on('disconnect', function () {
console.log('A user disconnected...');
});
console.log(album);
});
});

“Wait a minute!”, I hear you say, “No way a socket has an add-album event!” Well, not yet… Let’s check back in our front-end JavaScript again.

var socket = io();
artist: 'Led Zeppelin',
title: 'Led Zeppelin III'
});

Save it, browse to localhost again and surely will the console print the album. That’s pretty sweet!

But we’ll need more. After all, we want to receive data on our client! Of course we aren’t going to see any data unless we show it on our page.

Let’s first check out the server. This is really very easy. Ready?

io.emit('add-album', album);

Nice, we can simply use io.emit and all connections will get an update. So our client sends out an ‘add-album’ event to the server and the server sends an ‘add-album’ event back to the client. We could’ve used any name as event.

Let’s check the front-end. I’ve added in some AngularJS, which I’ve talked about before in MEAN web development #6: AngularJS in the front. Actually I’ve copied this example from that post and changed ‘book’ to ‘album’.

<html>
<meta charset="utf-8">
<title>Sockets example</title>
<script src="angular.min.js"></script>
<script src="/socket.io/socket.io.js"></script>
<script src="client.js"></script>
<body ng-app="socketsApp">
<div ng-controller="socketsController">

<h1>Sockets example</h1>
<ul>
<li ng-repeat="album in albums">
{{ album.artist + ' - ' + album.title }}
</li>
</ul>

<input type="text" ng-model="newArtist" />
<input type="text" ng-model="newTitle" />
</div>
</body>
</html>

And here’s the JavaScript.

angular.module('socketsApp', [])
.controller('socketsController', function ($scope) { var socket = io();$scope.newArtist = null;
$scope.newTitle = null;$scope.albums = [];
$scope.addAlbum = function () { socket.emit('add-album', { artist:$scope.newArtist,
title: $scope.newTitle });$scope.newArtist = null;
$scope.newTitle = null; }; socket.on('add-album', function (album) {$scope.$apply(function () {$scope.albums.push(album);
});
});
});

Notice the $apply function of AngularJS. It’s a detail, but I need it to execute the code in the AngularJS context so that the view gets updated immediately. What really matters is the socket.on function. I don’t have to explain it, you know how it works. It’s the exact same code you use on the server! Now open up two tabs, two browsers, two windows, two whatever, and browse to localhost. Enter an artist and title in one window and submit. Now switch to the other window and you’ll see the album you’ve just added in the other window! No refresh, no nothing! If you’re really using two windows put them next to each other and see the effect live in real time. Now maybe you’re thinking this is awesome, but there’s no need to send back the object to the client that just sent it to the server. For this you can use socket.broadcast.emit. This will send out a message to all sockets, except the one that’s broadcasting. socket.broadcast.emit('add-album', album); And the front-end would now look as follows. $scope.addAlbum = function () {
var album = {
artist: $scope.newArtist, title:$scope.newTitle
};
$scope.albums.push(album);$scope.newArtist = null;
$scope.newTitle = null; }; Pretty sweet! ## Rooms Let’s say you’re building that chat app we were talking about earlier (basically what we created here too). Now maybe you want your web site to have multiple chat rooms. A room for C# discussion, a room for JavaScript discussion, or maybe people can create their own rooms about whatever (maybe cars, music, movies). And perhaps you want to implement a private chat as well (one on one, or only invited people). How can you do this? I’ll leave the previous example for what it is and start a new one. From our page we want to be able to create a room, join a room, send a message and leave a room. So let’s check out the Node.js server side. io.on('connection', function(socket){ console.log('A user connected!'); socket.on('disconnect', function () { console.log('A user disconnected...'); }); socket.on('add-room', function (room) { console.log('Added:'); console.log(room); socket.join(room.name); socket.broadcast.emit('add-room', room); }); socket.on('join-room', function (room) { console.log('Joined:'); console.log(room); socket.join(room); }); socket.on('send-message', function (message) { console.log('Send:'); console.log(message); socket.broadcast.to(message.roomName).emit('receive-message', message); }); socket.on('leave-room', function (room) { console.log('Leave:'); console.log(room); socket.leave(room); }); }); That’s quite a bit of code, but there’s not much new, really! When someone adds a room we just broadcast the room to all other clients so everyone can see the new room. Additionally we ‘join’ the room using socket.join. This creates a group of sockets that can be notified at once. When a user joins a room we simply pass in a room name and again call socket.join. Then when someone sends a message to a room we simply call socket.broadcast.to(roomName) and now only clients that have joined that specific group will get a notification. That’s pretty easy, right? When we leave a room we simply call socket.leave and the client will stop receiving notifications from that specific room. The client-side scripting for this example isn’t really interesting. You can get the complete example from GitHub. And yes, I admit, the HTML could use a little work. The JavaScript works the same as in the previous example. We simply use socket.emit, to send events to the server, and socket.on to receive events from the server. I’ll leave it as practice for the reader to add user names, persist chat rooms, get a list of active rooms, implement private rooms, etc. If you want to do more with sockets and socket.io I can recommend reading Syncfusion’s Node.js Succinctly. It has a chapter on ‘vanilla’ sockets programming in Node.js (including an example on UDP) and a chapter on socket.io. If you’re still struggling with AngularJS I can recommend reading AngularJS Succinctly. Additionally Manning has a great book on Node.js, Node.js in Action, which, of course, also covers sockets. A second edition of the book is also in the making! Next week we’ll be wrapping up the MEAN web programming series! Happy coding! # MEAN web development #7: MongoDB and Mongoose Last week we’ve seen some of the basic functionality of AngularJS, at least enough to get you started. Before that we’ve seen Node.js and Express. So that’s EAN and we’re left with the M. Well, Dial M for MongoDB because that’s what we’re going to look at this week. As usual you can find the examples for this post on my GitHub page in the mean7-blog repository. ## Hello persistent data I’ve already written an entire post on NoSQL and MongoDB, A first look at NoSQL and MongoDB in particular. I’ve already told you to read it in the first part of this series, MEAN web development #1: MEAN, the what and why. If you haven’t read either of those I suggest you do so before continuing because I won’t repeat how to install MongoDB and MongoVUE. Don’t worry I’ll wait… Before we continue I should mention that everything we’re going to do is async. That means lots of callback functions. We don’t want to block our main thread after all! It also means that the callbacks may not be called in the same order as their ‘parent functions’ are. Or that the records are actually inserted before we query them! Because I wanted to keep it simple in the complete example file I haven’t nested all examples in callbacks, but keep in mind that you may get some odd results. It worked fine for me by the way, if you get some weird results try running the examples one by one (simply commenting out the others). So let’s just get a Node.js server up and running and write some data to the database real quick! You’ll be surprised how easy it is. First of all install the MongoDB driver using npm (npm install mongodb). Next we’ll make a connection to our MongoDB instance. var app = require('express')(); var MongoClient = require('mongodb').MongoClient; var urlWithCreds = 'mongodb://user:password@localhost:27017/local'; var url = 'mongodb://localhost:27017/local'; MongoClient.connect(url, function (err, db) { if (err) { console.log(err); } else { console.log('Connected to the database.'); db.close(); } }); var server = app.listen(80, '127.0.0.1'); So first of all we require Express (which isn’t necessary for MongoDB) and MongoDB. We take the MongoClient property of the MongoDB module. We use this client to connect to the database using the connect function, which takes a URI and a callback function. The callback has a MongoError (in case you can’t log in, for example if you have wrong credentials) and a Db as parameters. We can use the Db object to do all kinds of stuff like creating and dropping databases, collections and indices and do our CRUD operations (Create, Read, Update, Delete). Let’s insert a simple object. var url = 'mongodb://localhost:27017/local'; MongoClient.connect(url, function (err, db) { if (err) { console.log(err); } else { var artist = { name: 'Massive Attack', countryCode: 'GB' }; var collection = db.collection('artists'); collection.insertOne(artist); console.log(artist._id); db.close(); } }); As you can see we use the db parameter to get a collection (the MongoDB variant of a database table) using the collection function. If the collection does not exist it will create one automatically. We can then simply insert an object using the insertOne function of the collection. Now something funny has happened. After calling insertOne our artist object suddenly has an _id property. MongoDB uses this _id to uniquely identify objects. So that wasn’t so bad right? Let’s look at other CRUD functionality! ## CRUD with MongoDB So let’s retrieve the record we just inserted. We can do this using the findOne function of the collection. MongoClient.connect(url, function (err, db) { if (err) { console.log(err); } else { var collection = db.collection('artists'); collection.findOne({ name: 'Massive Attack' }, function (err, artist) { if (err) { console.log(err); } else { console.log(artist); } db.close(); }); } }); So the object that is passed to the findOne function is actually a search parameter. In this case we’re looking for documents (or records) that have a name equal to ‘Massive Attack’. The second parameter is a callback function that gives us an error, if any occurred, and the document that was retrieved. If you ran the previous example multiple times Massive Attack will be in your database more than once (having different values for _id), in this case findOne simply returns the first document it finds. So let’s insert a few more artists, just so we’ve got a little set to work with. We can use the insertMany function for this. collection.insertMany([ { name: 'The Beatles', countryCode: 'GB', members: [ 'John Lennon', 'Paul McCartney', 'George Harrison', 'Ringo Starr' ] }, { name: 'Justin Bieber', countryCode: 'No one wants him' }, { name: 'Metallica', countryCode: 'USA' }, { name: 'Lady Gaga', countryCode: 'USA' } ], function (err, result) { if (err) { console.log(err); } else { console.log(result); } });  Now you may think there’s a findMany function as well, but it’s actually just called find. find returns a Cursor which is something like an array, but not quite. We can use the toArray method though. The find function has a query parameter which is just an object that describes what fields of a document must have which values. We can search fields with AND, OR, NOT, IN, greater than, lesser than, regular expressions and everything you’re used to in SQL databases. var findCallback = function (err, artists) { if (err) { console.log(err); } else { console.log('\n\nFound artists:'); artists.forEach(function (a) { console.log(a); }); } }; // All documents. collection.find().toArray(findCallback); // Name not equal to Justin Bieber. collection.find({ name: {$ne: 'Justin Bieber' } }).toArray(findCallback);

// Name equal to Massive Attach or name equal to The Beatles.
collection.find({ $or: [{ name: 'Massive Attack' }, { name: 'The Beatles' }] }).toArray(findCallback); // Members contains John Lennon. collection.find({ members: 'John Lennon' }).toArray(findCallback); Now let’s update a record. collection.findOneAndUpdate({ name: 'Massive Attack' }, {$set: {
cds: [
{
title: 'Collected',
year: 2006,
label: {
name: 'Virgin'
},
comment: 'Best Of'
},
{
title: 'Mezzanine',
year: 1998,
label: 'Virgin'
},
{
title: 'No Protection: Massive Attack v Mad Professor',
year: 1995,
label: 'Circa Records',
comment: 'Remixes'
},
{
title: 'Protection',
year: 1994,
label: {
name: 'Circa'
}
}
]
}
}, function (err, result) {
console.log('\n\nUpdated artist:');
if (err) {
console.log(err);
} else {
console.log(result);
}
});

Here we see the findOneAndUpdate in action. Alternatively we could’ve used updateOne. And for multiple updates we can use updateMany.

Now let’s delete a record. There’s one guy I really don’t want in my database (yes, I’ve added him so I wouldn’t feel guilty about deleting him).  And for this we can, of course, use findOneAndDelete.

collection.findOneAndDelete({ name: 'Justin Bieber' }, function (err, result) {
console.log('\n\nDeleted artist:');
if (err) {
console.log(err);
} else {
console.log(result);
}
});

Really no surprises there. Alternatively there’s deleteOne and to delete many use, you guessed it, deleteMany.

## Mongoose

So MongoDB with Node.js looks really good, right? It wasn’t very hard to use. It’s really just a matter of working with JavaScript objects. And as we all know JavaScript objects are very dynamic. In the previous examples we’ve already seen that some artists have a member property defined and then when we updated we all of a sudden had a cds property and some CD’s have a comment while others don’t… And MongoDB has no problem with it at all. We just save and fetch what is there.

Now try this.

var app = require('express')();
var MongoClient = require('mongodb').MongoClient;

var Artist = function (name, activeFrom, activeTo) {
if (!(this instanceof Artist)) {
return new Artist(name, activeFrom, activeTo);
}
var self = this;
self.name = name;
self.activeFrom = activeFrom;
self.activeTo = activeTo;
self.yearsActive = function () {
if (self.activeTo) {
return self.activeTo - self.activeFrom;
} else {
return new Date().getFullYear() - self.activeFrom;
}
};
};

var url = 'mongodb://localhost:27017/local';
MongoClient.connect(url, function (err, db) {
if (err) {
console.log(err);
} else {
var collection = db.collection('artists');
// Empty the collection
// so the next examples can be run more than once.
collection.deleteMany();

var massiveAttack = new Artist('Massive Attack', 1988);
console.log('\n\n' + massiveAttack.name + ' has been active for ' + massiveAttack.yearsActive() +  ' years.');

collection.insertOne(massiveAttack);

collection.findOne({ name: massiveAttack.name }, function (err, result) {
if (err) {
console.log(err);
} else {
try {
console.log('\n\n' + result.name + ' has been active for ' + result.yearsActive() +  ' years.');
} catch (ex) {
console.log(ex);
}
}
});

}
});

var server = app.listen(80, '127.0.0.1');

What happens is that MongoDB doesn’t store the yearsActive function nor the constructor function. What MongoDB stores are just the non-function values. The result is that when we retrieve our object it will no longer be an Artist object, but just an object that just so happens to have the same properties as an Artist.

This is where Mongoose comes to the rescue! Mongoose adds a schema to your MongoDB objects. Let’s see how that works.

To add Mongoose to your project you can install it using npm install mongoose.

So first we can use mongoose.connect to get a connection to the database.

var app = require('express')();
var mongoose = require('mongoose');

var url = 'mongodb://localhost:27017/local';
mongoose.connect(url);
var db = mongoose.connection;
db.on('error', function (err) {
console.log(err);
});
db.once('open', function (callback) {
// ...
});

var server = app.listen(80, '127.0.0.1');

After that we can define a schema using the Schema function.

db.once('open', function (callback) {
var artistSchema = mongoose.Schema({
name: String,
activeFrom: Number,
activeTo: Number
});
artistSchema.methods.yearsActive = function () {
var self = this;
if (self.activeTo) {
return self.activeTo - self.activeFrom;
} else {
return new Date().getFullYear() - self.activeFrom;
}
};
});

And as you can see I’ve appended the yearsActive function to the artistSchema.methods object. After that we can create a Model using mongoose.model.

var Artist = mongoose.model('Artist', artistSchema);

And after that the Artist variable (a Model) is actually the portal to your collection. It’s also a constructor function for artists. So let’s create our Massive Attack artist.

var massiveAttack = new Artist({ name: 'Massive Attack', activeFrom: 1988 });
console.log('\n\n' + massiveAttack.name + ' has been active for ' + massiveAttack.yearsActive() + ' years.');

And then we can save it using the save function.

massiveAttack.save(function (err, result) {
if (err) {
console.log(err);
} else {
// ...
}
});

And now that the artist is saved let’s retrieve it and call that yearsActive function again. We can simply retrieve our object using Model.findOne.

Artist.findOne({ name: massiveAttack.name }, function (err, result) {
if (err) {
console.log(err);
} else {
try {
console.log('\n\n' + result.name + ' has been active for ' + result.yearsActive() +  ' years.');
} catch (ex) {
console.log(ex);
}
}
});

And here I’ve put the findOne directly in the callback function of save, which I didn’t do before. I needed this because calling findOne directly after save didn’t yield any results (timing issue I guess). More importantly it did successfully execute the yearsActive function!

And like with the regular MongoDB driver we can use find, remove, findOneAndRemove and findOneAndUpdate.

So we’ve looked at the MongoDB driver and at the problem of schemaless objects which Mongoose fixes. I can recommend practicing a bit, as it’s really very easy to drop, create, insert, update, read and remove data, and reading the API documentation of both. We’ve only scratched the surface here, but it got you on your way.

And of course I’m going to recommend some additional reading. The Node.js Succinctly book is just a great resource for Node.js in general and it has a tiny bit on MongoDB and SQLite as well. I can also recommend MongoDB Succinctly. And Getting MEAN from Manning even has a chapter on MongoDB and Mongoose.

Happy coding!

# MEAN web development #6: AngularJS in the front

Hi all and welcome back! Welcome back to me too as I’ve been away for a while. Let’s just say I was enjoying a summer vacation from blogging. But I’m back to finish this MEAN series! This week I’ve got AngularJS for you. Maybe next week too.
And in case you need a refresher, here are my previous posts about MEAN:

So in the previous post we’ve seen how we can build pages using the Jade template engine. Pretty sweet, but we need something bigger, better and badder for our front-end.

You can find the examples for this blog on my GitHub page in the mean6-blog repository.

## What is AngularJS?

You may have heard of AngularJS before. It’s one of the most popular open-source front-end JavaScript frameworks for the web developed by Google. And, of course, it’s the A in MEAN. But what does it do?

AngularJS is an MVVM framework, much like Knockout.js. It does a bit more than Knockout.js does though. Next to bindings AngularJS can be used for DOM manipulation, more like jQuery. And it does even more, like handling AJAX and routing. As Google likes to put it: AngularJS is “Superheroic” in that it does just about everything.

So instead of talking let’s see some action! First of all we need to install AngularJS. You can download it from the AngularJS website, or you can install it through a package manager such as npm or Bower (install angular). I’ve discussed all three methods in earlier posts so I will not repeat them here. Anyway, if you’ve downloaded my samples from GitHub you’ll be good to go.

The first examples of this post can be tested using nothing but the file system. For later examples with AJAX we’re going to need a little Node.js server, so I’ll add that later. The non-server examples can be found in the front-end.html and the front-end.js files.

So let’s take a look at a first example.

<html>
<meta charset="utf-8">
<title>AngularJS example</title>
<script src="node_modules/angular/angular.min.js"></script>
<body ng-app>
<p>This is your first angular expression: {{ 'This is AngularJS' + ' syntax!' }}</p>
</body>
</html>

So there are two things going on here. First is the ng-app directive in the body element. This tells AngularJS that body is the root element of our application and that AngularJS should do its magic. You can place it anywhere you want (like in the html element, or maybe a div element somewhere) and a page can have multple ng-app directives (which I won’t be doing in this post).

Next is, of course, the weird {{ }} syntax, which is the syntax AngularJS uses for its bindings. In this case you’ll see that ‘This is AngularJS’ and ‘ syntax!’ are, indeed, appended in your browser, like it was just some JavaScript. Try using {{ 1 + 2 }} and you’ll see it will print ‘3’ since 1 + 2 equals 3 in JavaScript (and not ’12’).

Now let’s look at something really very cool. Suppose you want to bind some value to an input. Here’s how to do it.

<body ng-app>
<p>Enter you name:
<input type="text" ng-model="firstName" />
<input type="text" ng-model="lastName" />
</p>
<p>You have entered: {{ firstName + ' ' + lastName }}</p>
</body>

No JavaScript required! Wow, that is so cool! And firstName and lastName are updated real time, while you’re typing! We bound our inputs using the ng-model directive which takes care of creating and binding the firstName and lastName properties.

## Enter controllers

You’ll often want more control over your code and putting all of your JavaScript into your HTML is not a good idea. So we’ll want to use AngularJS with some custom JavaScript file that we wrote. This is where things get tricky. Not very tricky, but you just got to know how it works.

An AngularJS application is defined by modules. Modules then define controllers.

angular.module('fullNameApp', [])
.controller('fullNameController', function ($scope) {$scope.firstName = '';
$scope.lastName = '';$scope.fullName = function () {
return $scope.firstName + ' ' +$scope.lastName;
};
});

So as we see here we create a module by calling the angular.module function and passing it more than one parameter. This returns an application so we can call the controller function directly on the return value. In the controller function we pass in the name of the controller, so we can use it in our HTML, and a constructor function, which receives a $scope variable to which we can append properties. Now our HTML would look like this: <html> <head> <meta charset="utf-8"> <title>AngularJS example</title> <script src="node_modules/angular/angular.min.js"></script> <script src="front-end.js"></script> </head> <body ng-app="fullNameApp"> <div ng-controller="fullNameController"> <p>Enter you name: <input type="text" ng-model="firstName" /> <input type="text" ng-model="lastName" /> </p> <p>You have entered: {{ fullName() }}</p> </div> </body> </html> So as you can see we’ve now named our ng-app directive and we’ve added a div element with an ng-controller directive which points to our fullNameController controller. Finally, we now use the fullName function. Notice that we could now set default values in our controller and they’ll be displayed on our page automatically. $scope.firstName = 'Sander';
$scope.lastName = 'Rossel'; ## More directives So let’s take a look at some more directives that can help you build amazing pages. ng-repeat can be used to repeat an element for every item in a list. So let’s add a little array on our controller. $scope.favoriteMovies = [
'Star Wars',
'Lord of the Rings',
'Fight Club'
];

And display it in an unordered list.

<ul>
<li ng-repeat="movie in favoriteMovies">
{{ movie }}
</li>
</ul>

Now we want to be able to add and delete items from the list. This next piece might blow your mind, but there’s a very simple directive to make a text input that can make a comma separated list and convert it to an array real time.

<input type="text" ng-model="favoriteMovies" ng-list />

So we bind the input to our favoriteMovies using ng-model and then use ng-list to convert it to a comma separated list. Try adding “, Pulp Fiction” (or whatever movie you like) to the text input and the movie will automatically be added to the unordered list we had before!
And if you want something else instead of a comma specifiy it in the ng-list, like so:

<input type="text" ng-model="favoriteMovies" ng-list=" | " />

$scope.favoriteAlbums = [ { artist: 'The Beatles', title: "Sgt. Pepper's Lonely Hearts Club Band" }, { artist: 'Moby', title: 'Play' }, { artist: 'The Prodigy', title: 'Fat Of The Land' } ]; <ul> <li ng-repeat="album in favoriteAlbums"> {{ album.artist + ' - ' + album.title }} </li> </ul> So how about making that editable? <div ng-repeat="album in favoriteAlbums"> <input type="text" ng-model="album.artist" /> <input type="text" ng-model="album.title" /> </div> <button ng-click="favoriteAlbums.push({})">New album</button> And that’s where we see the ng-click in action. Of course we could’ve called a function on our controller too. $scope.addAlbum = function () {
$scope.favoriteAlbums.push({}); }; <button ng-click="addAlbum()">New album</button> There’s a little problem with ng-repeat that isn’t quite obvious from these examples. ng-repeat creates a separate scope for each object in an array. For objects this is no problem, but if you were binding to an array of primitives, such as integers or strings, the values wouldn’t be updated. So just remember to use objects when wanting to update array elements. So how about adding a little styling to our page? With the ng-class directive we can add styles to elements based on some boolean value. The best part is you can add multiple classes based on one or more boolean values. So I have added a little embedded CSS to our page. <style> .colored { color: red; } .underlined { text-decoration: underline; } </style> And two properties in the controller to specify whether we want some text to be colored and/or underlined. $scope.addColor = true;
$scope.addUnderline = false; Now in our HTML we can add two checkboxes for the two properties above and then add an ng-class directive on some element that adds the CSS classes based on the JavaScript properties. Color: <input type="checkbox" ng-model="addColor" /> Underline: <input type="checkbox" ng-model="addUnderline" /> <p ng-class="{ colored: addColor, underlined: addUnderline }">This text might be colored and/or underlined!</p> Pretty sweet, right? As you can see ng-class should simply evaluate to an object where each property is a class name with a value true or false to specify whether it should be applied. But there’s an alternative. We could simply specify a string too. So consider the next JavaScript function which returns a string: $scope.getClasses = function () {
var classes = '';
if ($scope.addColor) { classes += 'colored '; } if ($scope.addUnderline) {
classes += 'underlined ';
}
return classes;
};

We can now use ng-class as follows:

<p ng-class="getClasses()">This text might be colored and/or underlined!</p>

Or you could return an array where each element is either an object such as in the first method or a string such as in the second method. I’ll leave that as practice for the reader though.

Also fun to mention, when you’re inside an ng-repeat directive you can use ng-class-even and ng-class-odd which work exactly as ng-class, but apply the styles only to even or odd elements.

## Filters

AngularJS had a feature called filters. It can format values or actually filter lists.

Let’s go back to the first example. Let’s suppose you wanted to show full name in just uppercase or in just lowercase. This is an easy task!

<p>You have entered: {{ fullName() | uppercase }}</p>
<p>You have entered: {{ fullName() | lowercase }}</p>

We can also use a filter to format numbers and dates. It looks kind of the same, except this time you throw in a format.

<p>
<input type="text" ng-model="number"><br>
Number (default): {{ number | number }}<br>
Number (no fractions): {{ number | number: 0 }}<br>
Number (three fractions): {{ number | number: 2 }}
</p>
<p>
Date (default): {{ date | date }}<br>
Date (dd-MM-yyyy): {{ date | date: 'dd-MM-yyyy' }}<br>
Date (yy/M/d): {{ date | date: 'yy/M/d' }}<br>
Date (full): {{ date | date: 'fullDate' }}<br>
Date (long): {{ date | date: 'longDate' }}
</p>

And for arrays you can do some awesome stuff too!  For this we use the orderBy or filter filter.

<h3>Ordered by title</h3>
<ul>
<li ng-repeat="album in favoriteAlbums | orderBy: 'title'">
{{ album.artist + ' - ' + album.title }}
</li>
</ul>
<h3>Ordered by artist reverse</h3>
<ul>
<li ng-repeat="album in favoriteAlbums | orderBy: 'artist' : true">
{{ album.artist + ' - ' + album.title }}
</li>
</ul>
<h3>Filtered on everything with '*y*'</h3>
<ul>
<li ng-repeat="album in favoriteAlbums | filter: 'y'">
{{ album.artist + ' - ' + album.title }}
</li>
</ul>
<h3>Filtered on artist with '*y*'</h3>
<ul>
<li ng-repeat="album in favoriteAlbums | filter: { artist: 'y' }">
{{ album.artist + ' - ' + album.title }}
</li>
</ul>

Keep in mind that we can get these values from our JavaScript controller as well. You can make the most awesome dynamic filters with very little trouble. Just make sure you pass in an object with the same properties as the objects you’re repeating and set their values to the values you’d like to filter.

## AJAX with AngularJS

So as mentioned AngularJS does a lot more than just HTML binding. One feature we’re going to look at here is that of the $http service. Needless to say we’ll need a little back-end server to communicate with, so for the AJAX examples I’m going to create a Node.js server and some new HTML and JavaScript so we can serve them up using our Node.js server. Here’s the server: var express = require('express'); var bodyParser = require('body-parser'); var app = express(); var books = [ { title: 'Lord of the Rings', author: 'J.R.R. Tolkien' }, { title: 'Harry Potter', author: 'J.K. Rowling' }, { title: "The Hitchhiker's Guide to the Galaxy", author: 'Douglas Adams' } ]; app.use(bodyParser.json()); app.use(express.static('public')); app.get(['/', '/index'], function (req, res) { res.sendFile(__dirname + '/public/ajax-example.html'); }); app.get('/books', function (req, res) { res.send(books); }); app.post('/addBook', function (req, res) { var book = req.body; books.push(book); res.send(books); }); var server = app.listen(80, '127.0.0.1'); So there’s a lot going on there. First of all I’ve added some Express. I’m also requiring body-parser (npm install body-parser), which is needed to get our POST data. Then I call the app.use function a couple of times. We’ve seen it before. It simply adds a middleware to a path. In this case we tell it to parse any JSON body so we can use it in our app. We also tell it that we can serve static files from the public folder (so we can serve the HTML and JavaScript file). After that we serve the page by browsing to the root or root/index. I’ve also specified a GET for root/books, which simply returns a list of books (as JSON). And, what it’s all about, a POST to add a book to the list of books. Let’s look at our front-end JavaScript. angular.module('ajaxApp', []) .controller('ajaxController', function ($scope, $http) {$scope.newAuthor = null;
$scope.newTitle = null;$scope.books = [];
$scope.getBooks = function () {$http.get('http://localhost/books')
.then(function (response) {
$scope.books = response.data; }, function (response) { alert('Something went wrong while getting the books!'); }); };$scope.addBook = function () {
$http.post('http://localhost/addBook', { author:$scope.newAuthor, title: $scope.newTitle }) .then(function (response) {$scope.newAuthor = null;
$scope.newTitle = null;$scope.books = response.data;
}, function (response) {
});
};
});

So that’s quite a thing too! First of all notice a $http parameter being passed to the controller constructor function. We’ll need this$http parameter to make HTTP calls. Now we first use this in the getBooks function. You’ll see that using this $http stuff is actually very simple! We use$http.get and pass it our URL to get the books. It returns a promise, which has a then function that takes two callbacks, a success function and a failure function. The functions are executed once the asynchronous call returns (AJAX is asynchronous, remember?). So if everything goes right we simply assign the data (a list of books) to the books property. If something goes wrong we show an alert.

In the addBook function I’m using the \$http.post function. It’s really similar to the get example, except we’re now passing in some data, a new book. Upon success the book list, including the new book, is returned and we reset the newAuthor and newTitle (nice example of two-way binding by the way).

Now for the HTML:

<html>
<meta charset="utf-8">
<title>AngularJS AJAX example</title>
<script src="angular.min.js"></script>
<script src="ajax-example.js"></script>
<body ng-app="ajaxApp">
<div ng-controller="ajaxController">

<h2>AJAX example</h2>
<button ng-click="getBooks()">Get books</button>
<ul>
<li ng-repeat="book in books">
{{ book.author + ' - ' + book.title }}
</li>
</ul>

<input type="text" ng-model="newAuthor" />
<input type="text" ng-model="newTitle" />
</div>
</body>
</html>

And there you have it!

So AngularJS can do two-way binding, supports MVVM and MVC architecture, it can do AJAX, but also routing, internationalization and localization, you can write custom directives and customize all you like, it supports testing and it has lots of directives I haven’t discussed like ng-show, ng-hide, ng-form, ng-blur… I guess Google wasn’t exaggerating when they called AngularJS ‘superheroic’ because it really seems to have some super powers!

And of course I’ll be leaving you with some recommended reading. As you know I’m a big fan of the (free) Succinctly series by Syncfusion, so here’s two must reads: AngularJS Succinctly and Node.js Succinctly. Also check out these awesome books by Manning Publications: AngularJS In Action, AngularJS In Depth, Node.js In Action (Second Edition) and Getting MEAN. And a book I’ve recently picked up at work: Pro AngularJS.
Well, that should keep you busy for a few months!

Happy coding!

# MEAN web development #5: Jade and Express

In the last few weeks it was my time to shine… In absence. Been quite busy with, you know, stuff. But here I am ready to give you a new installment of the MEAN web development series! If you haven’t read my previous posts, or if you’ve forgotten all about them in my absence, you can find them here:

So in my previous blog from a few weeks back I’ve talked about using Express in Node.js. One thing I haven’t discussed yet is the templating option of Express. As that is actually quite simple this blog will be more about the template engine Jade than about Express. Of course we’ll use both to build a few pages.

You can find the examples for this blog on my GitHub page in the mean5-blog repository.

So you can probably guess our first step (assuming you’ve already installed Node.js)… If it included npm somehow you are correct. We need to install Jade using npm. We’ll also want Express, of course, you know how it goes! You can read about npm and how to use it in my first MEAN article, MEAN web development #1: MEAN, the what and why. So create a folder for your project somewhere, optionally create a package.json file (containing at least an opening and closing bracket), open up a command prompt and use the following commands.

cd C:\project-folder
npm install express --save
npm install jade --save

The –save is optional and only makes sense when you created a package.json file.

Finally create your server.js file where your actual JavaScript goes. So now you have everything you need to start using Node.js, Express and Jade.

Let’s not do that yet though. Let’s take a step back. What exactly is Jade and why would you want to use it? Jade is a template engine, which means you can write some pseudo-HTML which the Jade engine can transform into actual HTML. One reason to use Jade is because it saves typing, “h1 Examples” will output “<h1>Example</h1>” for example. So that saves like seven characters, but it adds up.
Less typing is the least good reason to start using Jade though. When using Jade it is possible to pass some JavaScript object to your Jade code and build your page differently depending on your JavaScript. For example you might have an array and you want to iterate over the array to show a table on your page. Or maybe you just want to show the name of the logged in user in the top right corner.
Jade also supports and encourages a more object oriented style of writing your HTML. That’s awesome because you can now write a piece of HTML Jade once and use it everywhere.

Sounds good, right? But isn’t all of this already possible using PHP, Java or C#? Yeah, sure. And you can even do this in Node.js. Simply create a string containing your HTML, do some string concatenation, replaces, etc. No problem at all.
So why would you still want to use Jade? Because Jade makes it easy! All the string concatenation and replaces you’d have to do in PHP or “vanilla” Node.js is done for you. What’s left is a file that’s easier to read than the HTML it ultimately produces! And the good thing about Jade is that it has Node.js and Express support out-of-the-box!

I think that title says it all, no? We won’t be using Node.js and Express just yet though. In fact I want you to fire up a command prompt and install Jade again, this time globally so we can use it from the command prompt.

npm install jade -g

Now create a folder in the Node.js project we just created and call it jade-examples. We’re going to use this folder and some of the examples later with Node.js and Express. So in that folder create a file and call it hello-jade.jade. If you got my sources from GitHub you can find the file in jade-examples. Put the following code in the file.

html
body
p This is awesome!

So that’s maybe a bit much for a simple Hello World example, but I’m pretty sure you can figure out what HTML this will generate. One thing I’d like to point out is that Jade is whitespace sensitive. More specifically, you must use whitespaces to nest elements. That means that you got to be really careful with your spaces, they’re not just makeup!
I recently spent an evening debugging some Haskell code (which is also whitespace sensitive) because I had used spaces on one line and a tab on another. Try figuring that out, it all looks the same!
I recommend using spaces by the way, if your editor supports it look for the option that converts tabs to spaces and you’ll be safe.

So how to get some HTML out of this? With the command prompt, of course. So browse to the folder where you’ve put your file and use the following command.

cd file-folder
jade hello-jade.jade

Now check it out, it generated a hello-jade.html file. It’s not very readable though. Everything is put on a single line! Let’s fix that. You can use some options in the command prompt. We’re going to use -P (that’s a capital P).

jade hello-jade.jade -P

Looks a look better, doesn’t it?

<html>
<body>
<p>This is awesome!</p>
</body>
</html>

For multiline text you should use one of the following methods. Use a dot after your element or use pipes | on every single line.

p.
This is one line.
This is a second.
p
|This is one line.
|This is a second.
p
This won't work.
It just won't.

The HTML that is generated looks as follows.

<p>
This is one line.
This is a second.
</p>
<p>
This is one line.
This is a second.
</p>
<p>
<This>won't work.</This>
<It>just won't.</It>
</p>

As you can see Jade will happily generate syntactically correct, yet invalid HTML! Keep in mind that HTML isn’t whitespace sensitive so the above HTML will still render the two lines of text on a single line in your browser. If you need a seperate line you’ll have to insert a <br> element.

So now that we’ve got that out of the way let’s look at some other features of Jade. Of course you can add attributes to your elements.

h1(id="header") Attributes
p(class="text") This is an example using attributes.
p Here's a text input.
input(type="text")

I won’t show you the HTML output as you can generate it yourself or look at the files from my GitHub.

Here’s some other stuff we can do with Jade.

// This comment will be visible in our HTML.
//- This one won't (it's the hyphen!)
//- Ids are so common we have a shorthand!
//- Same goes for classes.
p.text This is an example using attributes.
p.text.second-class.third-class Here's a text input.
input(type="text")
//
Block comment
With lots of space
And lines

p Some text with another <strong>element</strong>.

//-
Yes, this is possible,
but we need to put it on a new line
and prefix it with a pipe.
| Some text without enclosing tags.

### Using JavaScript

So that’s all pretty cool, but you probably want more. You’re in luck, because there’s much more! In fact, you can use JavaScript to generate HTML. That means you can have any JavaScript object and use it just like that.

For the next example we’re going to need two files. First is our Jade file. I’ve called it javascript.jade.

h1= title
p= text

The = sign means some code will follow. So “title” and “text” is actually code. More precise they’re properties/variables. Using = will also filter your JavaScript to prevent injection attacks. If you like to disable this feature simply use != instead.

You could generate HTML from the above Jade file, but since title and text are not set you’ll just get an empty header and paragraph. So we’ll need to pass in something to the engine so it knows how to interpret those values. We can do that using a JSON file. So create another file, I’ve called it javascript.json, and put in the following JSON.

{
"text": "You can just insert JavaScript Objects into your templates!"
}

Now we can use the command prompt to generate the HTML using the JSON file. Notice that we’ll use a capital O in the next sample.

jade -O javascript.json javascript.jade

This will generate the HTML and it will have replaced title and text with the values from your JSON file. Awesome!

In the next example I’ve included all JavaScript into the Jade file so we don’t need a seperate JSON file. Try setting the showHeader variable to false and generate the HTML again. Also notice the difference between the escaped and the unescaped code.

//-
The following code
will not be shown
We'll use - instead of =

-

//- Single line code.

//- Multiline code.
-
var movies = [];
movies.push('Star Wars');
movies.push('Lord of the Rings');
movies.push('Harry Potter');

ol
each movie in movies
li= movie

- var code = 'HTML may look like this: <p>Hello, Jade!</p>';
//- Some escaped code.
p= code
//- Some unescaped code.
p!= code

### Inheritance

So how about we look into that inheritance now? Most, if not all, websites have a standard structure that’s the same across the entire page. We have a header at the top, footer at the bottom, a menu on the left, top or right and the content in the middle. And it’s usually the middle part that changes while the rest stays more or less the same. So let’s define that basic layout. I’m going to keep it simple so I’ll just have a page with a fixed footer. For simplicity I’m just going to use embedded CSS in our Jade template. I’ve called the following file base.jade.

html
style(type="text/css").
html {
position: relative;
min-height: 100%;
}

body {
margin: 0 0 100px;
}

footer {
background-color: #41a62a;
position: absolute;
left: 0;
bottom: 0;
height: 100px;
width: 100%;
}
body
div#content
p This is the content!
block content
footer
p This is the footer!
block footer

This looks a lot like what we already know. The only weird thing here are those blocks. The block elements aren’t visible in the generated HTML, so it’s completely possible to use a ‘base’ page as a regular page on your site.
Now those blocks can be overridden by inheritors. So block content can be replaced with anything you like. Let’s see what that looks like. I’ve created an base-inheritor.jade file which extends base.jade.

extends base.jade

title Inheritor

block content
p This is the inheritor page!

block footer
a(href="#") Some link

The file starts with extending base.jade (that’s actually the relative path to the file, since I have it in the same folder it’s just the file name). After that I’m filling in the content for each block. It’s totally optional to override a block.
Now you can simply generate the HTML for base-inheritor.jade and you’ll see the entire base is generated too!

There’s a little more you can do with those blocks though. Here’s a simple file called blocks.jade.

block header

block content
p Here is some content.

block footer
p This is the footer.

As you can see I’ve put some content in each of those blocks. In an inheritor we can replace that content, append to that content or prepend to that content. This is illustrated in blocks-inheritor.jade.

extends blocks.jade

prepend content
p This is even more content!

append footer
a(href="#") Some link

You may use “block append” or “block prepend”, but the block keyword is optional. Now generate the HTML and check out the results!

It’s also possible to put blocks in blocks, but I’ll leave that as practice for the reader.

### Mixins

Now there is one last feature I’d like to show you, which is mixins. A mixin is a bit like a function, a reusable piece of Jade! Let’s just look at some examples.

//- Here is a mixin.
mixin lorem
h2 First paragraph of Lorem Ipsum...
p Lorem ipsum [...]

h1 Mixins
//- Mixin usage is a big plus!
+lorem
+lorem
+lorem

h2 Animals
- var animals = ['Cat', 'Dog', 'Goldfish'];

//- A mixin with a parameter and JavaScript!
mixin isKnownAnimal(animal)
if animals.indexOf(animal) >= 0
p= 'The ' + animal + ' is a known animal.'
else
p= 'The ' + animal + ' is not a known animal!'

+isKnownAnimal('Cat')
+isKnownAnimal('Dog')
+isKnownAnimal('Parrot')

Pretty awesome, right?

## Express

So that’s all the Jade I’ve got for you! You can now create Jade pages and generate HTML through the command prompt. We’d use it within Node.js though! So the time has come to get back to the project we created at the start of this post.

In the server.js you need to do two things. Tell Express where to find its templates and what engine to use when rendering those templates. Then on a request render any template you like.
It’s that easy!

Now we’ve got all these Jade examples we can generate using Express! I picked a few interesting ones. Here’s the code for the entire server.js.

var express = require('express');
var app = express();

app.get('/', function (req, res) {
});

app.get('/javascript', function (req, res) {
res.render('javascript', {
text: 'You can just insert JavaScript Objects into your templates!'
});
});

app.get('/base-inheritor', function (req, res) {
res.render('base-inheritor');
});

app.get('/mixins', function (req, res) {
res.render('mixins');
});

var server = app.listen(80, '127.0.0.1');`

Is that all? That’s all!

So I usually give you some reading recommendations at the end of a blog, but I really haven’t got anything for Jade. You can get a lot from the language reference on the Jade website, although it’s hardly a good place to start.
Of course you can always check out some books by Manning, like Express.js in Action, which has a chapter on templating in Express and Jade. And keep an eye out for the ebooks by Syncfusion. Node.js Succinctly has a chapter on Express and JavaScript Succinctly is always handy when working with JavaScript.

Hope to see you again next week.
Stay tuned!