Why does re-declaring an argument inside of a try/catch throw a ReferenceError?

Question

I mistakenly wrote a re-declaration of an argument as a const in a function and instead of throwing SyntaxError: Identifier \'bar\' has already been decla

Answer 1

It's a myth that const and let aren't hoisted at all. They're half-hoisted. :-) That is: Within a block, if const bar or let bar (or class bar { } for that matter) appears anywhere in that block, then bar cannot be used prior to that declaration in the block — even if it exists in a containing scope. This area between the beginning of the block and the declaration is called the Temporal Dead Zone:

function foo(bar) {
  // `bar` is fine here
  try {
      // Temporal Dead Zone, `bar` cannot be used here
      console.log(bar);
      // End of TDZ
      const bar = 123;
      // `bar` would be fine here
  } catch(err) { console.log(err) }
}
foo(456);