Defined variable in outer scope is not defined in inner scope [duplicate]

Answer 1

This is caused by hoisting the internal let or const to the top of the block (MDN), and create a Temporal Dead Zone.

In ECMAScript 2015, let will hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

The reason behind this is simple - silent failing vs throwing error.

In this example, a similar setting is giving the result as undefined because of the var hoisting. This is a silent error, and it might be very hard to debug.

{
    var a = 'a1';
    (function() {
        console.log(a);
        var a = 'a2';
    })();
}

If you'll use let or const an error will be thrown:

{
  let a = 'a1';
  (function() {
    console.log(a);
    let a = 'a2';
  })();
}

You can find more info about the temporal dead zone in the article TEMPORAL DEAD ZONE (TDZ) DEMYSTIFIED.