How JS works for multiple variables with same name?

Question

Why there is no Syntax error in the following code?

var num = 8;
var num = 10;
console.log(num) // 10

whereas

var num = 8;
let num = 10;
console.log(num) // already decleared error

Answer 1

The first case will be rendered as:

var num;
num = 8;
num = 10;
console.log(num); // output : 10

The second case will be rendered as:

var num;
num = 8;
let num; // throws Uncaught SyntaxError: Identifier 'num' has already been declared
num = 10
console.log(num);