JavaScript Variables

A variable contains a value. When you use the variable, you refer to the data it represents. You use variables to store, retrieve and manipulate values in your code.


Declaring Variables

In the declaration it's where a variable appears in the script for the first time. The declaration sets it up in the memory so that you can refer to it in your script. A variable should be declared before start using it. To declare it you will do this by using the var keyword.

JavaScript - Declaring variables

<!DOCTYPE html>
    <html>
        <body>
         
        <h2>JavaScript Variables</h2>
         
        <p id="variablesdemo"></p>
         
        <script>
        // A single declaration.
        var x;
         
        // Multiple declarations with a single var keyword.
        var x, y, z;
         
        // Variable declaration and initialization in one statement.
        var x = 0, y = 100;
        var z = 234;
        var x = 5;
        var y = 6;
        
        // Let's do some calculation.
        z = y * x;
         
        document.getElementById("variablesdemo").innerHTML = "The value of z is: " + z;
        </script>
         
    </body>
</html>

Learn more about JavaScript Var Learn more about JavaScript Let

Naming Variables

Like many other programming languages also JavaScript is case-sensitive. That means that the variable oldCar is not the same variable as the variable OLDCar. A variable name can consist of any number of characters. There are some rules to be followed when we create a legal variable name:

  • It's required to use an ASCII letter as the first character, it can be either an uppercase or a lowercase. It must comply with Unicode variable naming conventions or it can be an underscore (_) character. Note that a number cannot be used as the first character.
  • All subsequent characters must be either letters, numbers, or underscores (_).
  • The variable name cannot be a reserved word.

Example of legal variable names:

var _oldDog
var OldDog
var old_Dog


Example of illegal variable names:

// Cannot begin with a number.
var 55old_Dogs

// The ampersand (&) character is not a valid character for variable names.
var Old & Dog


When a variable is declare and initialized it can be assigned to the value null. This is done if you don't want to give it any particular value.
Here is an example:


var x = null;
 
var y = 3 * x;  // y = 0

When a variable is declared without assigning a value to it, it will have the value undefined.

Take a look at this example:


var x;
 
// y will have the value NaN because x is undefined.
var y = 1 * x;
 
// Result: NaN
alert(y); 

When a variable is declared without using the var keyword in the declaration, we call that an implicit declaration. It is possible to assign a value to it at the same time.


// Here the variable x is declared implicitly.
x = "";
 
// This will return an empty result.
alert(x);

It is not possible to use a variable that is not declared. That will cause an error.


// This will cause an error since the variables are not declared and do not exist.
var x = a * b;
 
alert(x);


Coercion

JavaScript is a loosely typed language and that means that JavaScript variables have no predetermined type. The type of a variable will be the same as the type of of its value. Because of this you can treat a value as if it were of a different type.

JS JavaScript - Coercion

var x = 2000;
var y = "Hello";
 
// The number is coerced to a string.
x = x + y;
 
document.write(x);
 
// Output:
// 2000Hello

Learn more about JavaScript Comments Learn more about JavaScript Scope
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JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

3.      JavaScript to program the behavior of web pages.

JavaScript is a scripting or programming language that allows you to implement complex things on web pages. Every time a web page does more than just sit there and display static information for you to look at, displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. − you can bet that JavaScript is probably involved.


The benefits of using JavaScript are:

1.      You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

2.      Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

3.      Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

4.      Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.


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