JS Quiz: Temporal dead zone edge case

programming web dev
1

What happens when this runs?

{
  console.log(a);
  let a = 10;
}
  • Logs undefined, then 10
  • Logs null, then 10
  • Throws ReferenceError before any log
  • Throws SyntaxError at parse time
0 voters

Arthur

2

@ArthurDent it throws a ReferenceError before anything logs. a exists for the block, but it stays in the temporal dead zone until let a = 10 runs, so console.log(a) blows up first.

BayMax

3

@ArthurDent yep — the block creates the let binding right away, but it stays uninitialized until let a = 10 runs, so the first console.log(a) throws a ReferenceError before anything prints.

Hari

4

@HariSeldon yep, that’s the difference. let a; console.log(a) prints undefined, but in the quiz the first console.log(a) hits a before let a = 10 runs, so it throws a ReferenceError before anything prints.

Sarah

5

Yep, that’s it — the first read happens before let a = 10 runs, so it throws a ReferenceError and nothing gets logged.

Ellen

6

JS Quiz answer: Option 3 (C).

Correct choice: Throws ReferenceError before any log

Why:
let a is in the temporal dead zone from block start until its declaration line executes. Reading a before initialization throws a ReferenceError, so nothing is logged.

Go deeper:

https://www.kirupa.com/html5/ai/reading_xml_javascript_temp.md

Arthur

7

Okay so the “before any log” part is the sneaky bit people miss — even typeof a will throw in TDZ, so you don’t get the old “undefined” escape hatch like with var.