Issue with let keyword

Question

I was playing with some code and ran into a situation where I couldn't identify why 'let' is behaving the way it does.

For the below block of code:

var x = 20; // global scope

function f() {
  let x = x || 30;
}

f(); // VM3426:1 Uncaught ReferenceError: x is not defined(…)

I get the error 'x is not defined' on executing f(). I do understand 'let' variables are not hoisted but since 'x' has a global copy, why isn't the line inside function 'f' defaulting to global copy instead of throwing an error? Does 'let' set the variable to undeclared (instead of 'undefined' with var because of hoisting) at the beginning of the function? Is there any way to get the global copy of 'x' within the function?

Answer 1

The exception is about the right side x - when you are initializing block-scope x variable the global one is already "forgotten" however the new one is still not being declared and cannot be used during initialization

Compare with explicit calling global one

    function f() {
      let x = window.x || 30;
    }

Also take a look at this MDN article about let dead zones