Does 'let' override a global declaration and throws a ReferenceError?

Question

I was going through the Difference between var and let documentation example and was testing that when an undeclared variable is invoked, the global scope automatically provides a declaration for it (that's why the following snippet does not throw an error in any of the variables):

x = 3;
console.log(x);

(function() {
  y=x+39;
})()
console.log(y);

However, when one variable is declared with let after the assignment in the same global scope:

x=3;
let x = 42;
console.log(x);

One of the following errors is thrown:

ReferenceError: x is not defined (Chromium)

ReferenceError: can't access lexical declaration x before initialization (Firefox)

I understand that let does not allow x to hoist, but since it was previously referenced (implying an automatic declaration from the global scope) shouldn't in this case a re-declaration happen?

SyntaxError: Identifierx has already been declared

And therefore the error above thrown?

I also understand that in strict mode the first snippet would throw a ReferenceError, so does this mean that let forces this particular rule of strict mode (all variables need to be declared) upon the global scope?

Answer 1

Did you have a look at the let docs at MDN? They describe a temporal dead zone and errors with let.

ES6 does hoist a let variable to the top of its scope. Differently to var variable, when using let you must not access the variable before it is declared. Doing so fail with a ReferenceError (a.k.a. let's temporal dead zone).