Always declare JavaScript variables with var,let, or const. Var , let, const all three are use to create variable in javascript but there are following differences.

Var

The var keyword is used in all JavaScript code from 1995 to 2015.
The let and const keywords were added to JavaScript in 2015.
If you want your code to run in older browsers, you must use var.

if(10>5)
{
  var a = 2;
}
console.log(a); // 2 it will work 

because a is defined using var

Let

The let keyword was introduced in ES6 (2015).
Variables defined with let cannot be Redeclared.
Variables defined with let must be Declared before use.
Variables defined with let have Block Scope.

if(10>5)
{
  let a = 2;
}
console.log(a); // error

a can be access only within {} because it is declared using let

Const

The const keyword was introduced in ES6 (2015).
Variables defined with const cannot be Redeclared.
Variables defined with const cannot be Reassigned.
Variables defined with const have Block Scope.

if(10>5)
{
  const a = 2;
  a = 100 // error because it is const
}
console.log(a); // error

because a is defined as const it scope is only within {}

Must be Assigned

JavaScript const variables must be assigned a value when they are declared:

const PI = 3.14159265359;