JavaScript Variables

JavaScript Variables

A variable is a way of storing and keeping track of information in a program. For example, a computer game keeps track of a player's score. At the start of the game, the score is zero. It can go up if the player does well or go down if the player makes a mistake. The game might even end if the player reaches a particular score. The score is an example of a variable. Although the score's value will change, zero at the beginning, 100 at the end, for example, it's still always just one score for that player.

A JavaScript identifier must start with a letter, underscore (_), or a dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

Declaring JavaScript variables

You can declare javascript variable in three different ways:

Using var keyword .

Example var x = 100;

This syntax can be used to declare both local and global variables.

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Variables in Javascript behave same as most popular languages like c, c++ and other languages variables. The only difference from other languages is, JavaScript variable is not mandatory to define the datatype of the variable. JavaScript will take care of all types even if you do not want define data type.

By Simply Assigning

Example x = 10;
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This always declares a global variable and cannot be changed at the local level. It generates a strict JavaScript warning. You shouldn't use this variant.

Using Keyword let

Example let y = 10;
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This syntax can be used to declare a block scope local variable

Declaring JavaScript Variables

JavaScript variables can be delcared using the variable name like below

Syntax var myname;

If you notice the above example the variable has no value. Technically it has the value of undefined

To assign a value to the variable, use the equal sign:

var myname = "John";

Declaring more than one variable

You can also declare more than one variable in the same line like below

var person = "John Doe", carName = "Volvo", price = 200;

var name = "John", designation = "Assistant Manager", price = 200; document.getElementById("demo").innerHTML = designation;
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JavaScript Identifiers

JavaScript Identifiers are named; names that you give things in JavaScript. These JavaScript “things” include

• variables
• functions
• objects
• properties
• methods
• events

There are some rules to follow while using the JavaScript language

• Identifiers can only contain letters, numbers, underscore (_) and the dollar sign ($).
• Identifiers cannot start with a number.
• Identifiers are case-sensitive.
• Identifiers can be any length.
• Identifiers can not be the same as JavaScript reserved words. (See list of reserved words)
• Don't use global properties and methods as identifiers (more later).
• Don't use words similar to reserved words.

When naming an identifier with two words in it, it's a best practice to use camel case. With this convention, the first letter of each word, excluding the first word, is uppercase. Example:

firstName myCommonVariableName

Here are some examples of valid identifier naming conventions:

firstname totalPrice cust_1 click_calculate $ $total

Note : JavaScript identifiers are case-sensitive.

JavaScript Data Types

JavaScript variables can hold different data types like numbers, string, and complex data types as well. In this javascript language, we call text related values as strings.

Lets look at the how numbers and strings can be used in JavaScript program. Here strings are written within the single or double quotes whereas numbers can be written without quotes.

Note:

numbers can be treated as strings if you mention quotes around it.

Let's see some example of numbers and strings

var name= "Williams"; // name is a string var jobRole = "Software Engineer"; var salary = 40000; // salary is a number
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Declaring JavaScript Variables before using it

As a JavaScript developer, you can always declare variables before you use them. Variables don't have to declare before being used

Example var firstVar = "Welcome to "; secondVar = firstVar + "SnoopCode"; // decaring secondVar on the go alert(secondVar);
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When JavaScript sees a declared identifier (var varName), it creates a global variable.

Reassigning value to Variables

JavaScript gives more flexibility to reassign or change the value of a variable anywhere in the middle of the program. When the program runs the latest value gets executed

Example var myScore = 78; alert("Initial myScore variable value is : " + myScore); // current value is 78 myScore = 81; alert("myScore variable value changed to: " + myScore); // value changed to 81 myScore = 96; alert("myScore variable value changed to: " + myScore); // variable value reassigned 96 myScore = 100; alert("myScore variable value changed to: " + myScore); // variable again reassigned to 100 // print the finally value of myScore variable document.writeln("The final value of myScore is" + " " + myScore);
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Sources and Credits

The source of the content has been referred and updated with Mozilla Developer Network and W3C Organization

Last Updated Jul 29, 2015, 07:00:14 PM