Don't we use let instead of var now

const is for constants that is, variables who's values should not change. In more detail, when you assign a value to const, you cannot change its type and value later. However, if the const variable is set to an array or object, the contents within can be modified but the same variable cannot be changed to a different type.

let works similar to var except it is block scoped. example:

// this will not do what you expect it to do.
// this will print 0 - 9 one time
for (var i = 0; i < 10; i++) {
  console.log(i);
  for(var i = 0; i < 10; i++) {
    console.log(i);
  }
}
// this will behave correctly
// with will print 0 - 9 nine times. This is how most programming languages behave.
for (let i = 0; i < 10; i++) {
  console.log(i);
  for(let i = 0; i < 10; i++){
      console.log(i);
  }
}

Block scope is good. It mean that the variable declared within a block of code does not leak outside of that block.

If you are working with modern browsers, use let. If you are using older browsers, use var. If you are using a REPL console, var is convenient.