Object-Oriented JavaScript:

Variables are used to store data. When writing programs, it is convenient to use variables instead of the actual data, as it's much easier to write pi instead of 3.141592653589793 especially when it happens several times inside your program. The data stored in a variable can be changed after it was initially assigned, hence the name "variable". Variables are also useful for storing data that is unknown to the programmer when the code is written, such as the result of later operations.

There are two steps required in order to use a variable. You need to:

In order to declare a variable, you use the var statement, like this:

var a; 
var thisIsAVariable; 
var _and_this_too; 
var mix12three;

For the names of the variables, you can use any combination of letters, numbers, and the underscore character. However, you can't start with a number, which means that this is invalid:

var 2three4five;

To initialize a variable means to give...

Operators take one or two values (or variables), perform an operation, and return a value. Let's check out a simple example of using an operator, just to clarify the terminology.

>>> 1 + 2

3

In this code:

Instead of using the values 1 and 2 directly in the operation, you can use variables. You can also use a variable to store the result of the operation, as the following example demonstrates:

>>> var a = 1; 
>>> var b = 2; 
>>> a + 1 

2

>>> b + 2 

4

>>> a + b 

3

>>> var c = a + b; 
>>> c 

3

The following table lists the basic arithmetic operators:

>>> 1 + 2

>>> 99.99 – 11

>>> 2 * 3
 6

Any value that you use is of a certain type. In JavaScript, there are the following primitive data types:

Any value that doesn't belong to one of the five primitive types listed above is an object. Even null is considered an...

Let's quickly summarize what has been discussed so far:

Now that you know the basic primitive data types in JavaScript, it's time to move to a more interesting data structure—the array.

To declare a variable that contains an empty array, you use square brackets with nothing between them:

>>> var a = [];
>>> typeof a; 

"object"

typeof returns "object", but don't worry about this for the time being, we'll get to that when we take a closer look at objects.

To define an array that has three elements, you do this:

>>> var a = [1,2,3];

When you simply type the name of the array in the Firebug console, it prints the contents of the array:

>>> a 

[1, 2, 3]

So what is an array exactly? It's simply a list of values. Instead of using one variable to store one value, you can use one array variable to store any number of values as elements of the array. Now the question is how to access each of these stored values?

The elements contained in an array are indexed with consecutive numbers starting from zero...

Conditions provide a simple but powerful way to control the flow of execution through a piece of code. Loops allow you to perform repeating operations with less code. Let's take a look at:

{
  var a = 1; 
  var b = 3;
}

{
  var a = 1; 
  var b = 3;
  var c, d;
  {
    c = a + b;
    {
      d = a - b;
    }
  }
}

One last thing for this chapter: comments. Inside your JavaScript code you can put comments. These are ignored by the JavaScript engine and don't have any effect on how the program works. But they can be invaluable when you revisit your code after a few months, or transfer the code to someone else for maintenance.

Two types of comments are allowed:

Some examples:

// beginning of line
var a = 1; // anywhere on the line
/* multi-line comment on a single line */	
/* 
    comment
    that spans
    several lines
 */

There are even utilities, such as JSDoc, that can parse your code and extract meaningful documentation based on your comments.

In this chapter, you learned a lot about the basic building blocks of a JavaScript program. Now you know the primitive data types:

You also know quite a few operators:

Then you learned how to use arrays to store and access data, and finally you saw different ways to control the flow of your program—using conditions (if-else or switch) and loops (while, do-while, for, for-in).

This is quite a bit of information and it is recommended that you now go through the exercises below, then give yourself a well-deserved pat on the back before diving into the next chapter. More fun is coming up!