What is the purpose of the let keyword?

The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword are used to define a variable globally, or locally to an entire function regardless of block scope.

Let's take an example to demonstrate the usage,

let counter = 30;
if (counter === 30) {
let counter = 31;
console.log(counter); // 31
}
console.log(counter); // 30 (because the variable in if block won't exist here)