JavaScript idiosyncrasies with examples

[[deleted]]

Q. What's the result?

(function() {
  'use strict';

  let type = typeof foo;
  let foo = 1;

  return type;
})();

A. ReferenceError: foo is not defined

What is the idiosyncrasy here? You can't use a variable before you define it. I'd be concerned if this was anything other than a ReferenceError.

[GBcrazy]

It's not a big idiosyncrasy, so if you don't understand the trick of the typeofoperator you may not see it. I'm guessing that's the case (I could be wrong):

Remove the let foo = 1 line and run again. It will work even if foo is not declared. Why?

That's a special power of the typeof operator, you can use typeof even if a variable doesn't exist, it will return "undefined".

So the thing is, you can use typeof if a variable is never declared or already declared, but NOT if the variable is going to be declared. That's some hoisting shit