JavaScript Variables are containers used to handle(store) values.
JS-Variable Identifier
Identifiers are rules for declaring variables. The following are identifiers for declaring JS variables.
  1. Variable name must start with a letter (a to z or A to Z), underscore (_), or a dollar sign($).
  2. Variable names can contain letters, digits, underscore, and dollar signs.
  3. JS variables are case sensitive (for example z and Z are different variable names).
  4. Reserved words or JS keywords are can not be used as variable names.

We can declare JavaScript variables in three ways.
1) Declaring Variables Using var Keywords
var keywords is a common way of declaring JS variable.
Example1: Correct variable declaration.


var a1=15;
var _b=10;
var c="DG Teck";
Example2: Incorrect variable declaration. 
var 1a=15;
var 32_b=10;
var 111="DG Teck";
In the above Example1, all variables are declared correctly by keeping JS variable identifiers. but, in Example2 all variables don't follow JS variable identifier rule, both variables start with digits.

In JavaScript the equal sign(=) is an assignment operator, used to assign a value for a variable. It is different from Algebra, If you want to use equal to an operator like algebra in javascript you can use double equal("==") sign.
Many variables in a Single line
To avoid unnecessary repetition, and reduce wastage of space, you can declare many variables and assign values in a single line by separating them with a comma.

var F_name="Minilik", School="hope School", Age=14;

Redeclaring Variables
var keyword allows you to redeclare variables again. In this case, you can also reassign values for variables. In the below example the compiler takes the second value ("10") for variable a.
 Example3: Redeclaring variables
<!DOCTYPE html>
<html>
<body>
	<h3>Redeclaring variables</h3>
	<p id="pa"></p>
<script>
	var a=18;
	var a=10;
	document.getElementById("pa").innerHTML = "the value of a ="+ a;
</script>
</body>
</html>
Output

Redeclaring variables


2) Declaring Variables Using let Keywords
when variables are declared with let can not be redeclared in the same block. variables defined with let has block scope.
 Example1: variable defined with let can not be redeclared in the same block as follows.
{
let a=18;
let a=10; //'a' has already been declared. you can not redeclare here!
}

let and const keyword provides block scope in JS. You can not access variables out of a block scope or {}. This means you can redeclare variables defined by let keywords out of block scope or {}.
Example2: You can not access variables out of block scope
<!DOCTYPE html>
<html>
<body>
	<h3>we can't access variable out of block scope</h3>
	<p id="pa2"></p>
<script>
	{
	let b=18;
	}
	document.getElementById("pa2").innerHTML = "the value of b ="+b;
</script>
</body>
</html>
Output

we can't access variable out of block scope

As you saw in the above output there is no displayed value of variable b because variable be is defined with let keyword so it can't be accessed out of block scope or {}.

3) Declaring Variables Using const Keywords
When you define variables with const, they can't be redeclared, reassigned and it has block scope then it can't be accessed out of block scope.
A JavaScript const variable must be assigned when it is declared. 
Example1: Can't be reassigned.
const a=18;
  a=10;//Error can't be reassigned
  a=30;//Error can't be reassigned
Example2: Variable redeclaring 
const a=18;
   a=15;//not allowed
var a=10;////not allowed
let a=30;////not allowed