Best Practice:Do not use var in for/while loop, because after the loop, it will hold the value due to it’s availability.
var can be considered to be defined a global variable in the function scope.


var keyword
Variables declared with the var keyword can not have Block Scope.
Variables declared inside a block {} can be accessed from outside the block.
Variables can be referred to before the declaration.

Example1:
{ 
  var x = 2; 
}
// x CAN be used here

Example2:
document.write( x );//undefined
var x = 9;
document.write( x );//9


let keyword
Variables declared with the let keyword can have Block Scope.
Variables declared inside a block {} cannot be accessed from outside the block.
Variables cannot be referred to before the declaration.
Example1:
{ 
  let x = 2;
}
// x can NOT be used here


Example2:
document.write( x );//throw an error
let x = 9;
document.write( x );//9