difference between let and var

In script you can declare a variable with multiple time.

for example 

var a=20;

var a=30;

console.log(a); //30

but with let you cant redeclare a same variable name.

exp

let data=20;

let data=30;

console.log(data);

//you will get error.

Identifier 'a' has already been declared.

With let you can reassign variable 

like 

let data=20;

data=40;