Variables in Javascript

A variable is storage area used to store a value in memory. To create a variable in javascript, var keyword is used. var is a reserved keyword in javascript, used to create variables. We can also create a variable without using var in javascript, but it is not recommended.

Javascript variables naming rules


 i=3;                           // variable declaration(Not Recommended);
 var x="Tech Altum";    	// value of x is "Tech Altum"; 
 var y='Tech Altum';    	// value of y is also "Tech Altum"; 
 var z=9;               	// value of z is 9 numeric.
 var a=2, b=3, c=5;     	// multiple variables in single line
 var u;				// u is undefined


Variable naming convention in JavaScript

To declare a variable in javascript, use following rules.

  • variables names cannot include javascript reserved keywords, like var, for, in, while, true, false, null, class, let, const, function etc.
  • variables cannot starts with numbers and hyphen, use alphabets or underscore(_).
  • variables can have strings followed by numbers, like x1 and x2 are allowed but 1x, 2x is not allowed.
  • For separation in variable names, use underscore (_) or camel casing, do not use hyphen (-) separation and white space, i.e, user_name is valid, username is also valid, but user-name is invalid.
  • variables names are case sensitive, i.e, x1 and X1 are different.

Strict Mode

ECMAScript 5 includes strict mode in javascript. "use strict" string is used to follow strict mode. In strict mode, var is compulsory, but in sloopy mode, we can also create variables without var keyword.

 "use strict";
 var x1="tech altum";       	 // allowed in strict mode
 x2="javascript";        	 // not allowed in strict mode
 for( var i=1;i<=10;i++){}       // allowed, 
 for( i=1;i<=10;i++){}           // not allowed, i is not declared
 var x=10;               // allowed
 var y=010;              // not allowed, it is octal notation.

Sloppy Mode

By default javascript is written in sloppy mode. Sloppy mode allow us to create a variable with or without var keyword.

 var x1="tech altum";       	 // allowed 
 x2="javascript";        	 // also allowed 

let and Const

let and const were introduced in ECMA 6. let is used to declare temporary values with limited scope, like inside a function. Whereas const is used to declare permanent values, which can't be change. Like value of pi, e, G and other constants.

let and const in javascript

 let x="user";       	 		// to store temporary values
 const pi=Math.PI;        	 	// pi is a constant, and can't change

Although it is not compulsory to start variables with var keyword, but still we prefer var. let and const are supported in IE 11 and above, edge, firefox 36 and above, chrome 21 and above, safari 5.1 and above.