Variables

What is a variable?
In javascript, all values are stored at some memory location.
Now, a user can not directly access a memory location hence variables are used.

A variable is a user defined name that is used to access a memory location. Using a variable, you can store or modify a value at the memory location.
Example,
Above illustration declares two variables(digit and lang) pointing to different memory locations having different values.
How variables are declared?
Variables in javascript are declared using var keyword followed by the name of variable which can be any user defined name.
Example,

var digit = 5;
var lang = 'javascript';

ES6(or ECMAScript6) introduced a different way of declaring variable.
It suggests using let keyword in place of var.

Thus, above variable declarations can be modified as below.

let digit = 5;
let lang = 'javascript';

If the variable value will never change after initialized for the first time or you do not want it to be changed accidentally, then use const in place of let.

Javascript engine will not allow modification in variable values declared with const. Example,

const digit = 5;
digit = 10;   // Illegal

Variable naming rules
Though a variable can be a user defined name but there are few rules that should be followed while declaring a variable.

They are
1. A variable name can not begin with a number.
Thus, 12digit, 0lang are invalid variable declarations. Variable names should begin with a letter or underscore. Examples, _start, digit123, technology etc., are valid variables.

2. A variable name can not be a javascript keyword or a reserved word(explained below).

Keywords
A keyword, also called reserved word, is a word which has some special meaning in the language syntax.
When a keyword is encountered, javascript engine performs some action that is specific to that keyword.
Example, break is a keyword.
When javascript engine finds this keyword, it terminates an executing loop.

A keyword cannot be used as a variable name, function or class name in javascript.
Following are the keywords in javascript.

abstract	boolean	break	byte	case	catch
char	class	const	continue	debugger	default
delete	do	double	else	enum	export
extends	false	final	finally	float	for
function	goto	if	implements	import	in
instanceof	int	interface	long	native	new
null	package	private	protected	public	return
short	static	super	switch	synchronized	this
throw	throws	transient	true	try	typeof
var	void	volatile	while	with