JS Quiz: Easy: Var vs let in timer loops

programming web dev
1

What sequence is logged?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}
for (let j = 0; j < 3; j++) {
  setTimeout(() => console.log(j), 0);
}
  • 3 3 3 0 1 2
  • 0 1 2 0 1 2
  • 3 3 3 3 3 3
  • 0 1 2 3 3 3
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2

3 3 3 0 1 2.

var i is one function-scoped variable, so by the time the timeouts actually run the loop’s done and i is already 3. let j is block-scoped and each iteration gets its own binding, so you see 0 1 2 even with a 0ms timeout (it still waits for the current stack to finish).

3

JS Quiz answer: Option 1 (A).

Correct choice: 3 3 3 0 1 2

Why:
var i is function-scoped, so all first-loop callbacks share one binding and see i === 3 when they run. let j is block-scoped per iteration, so the second-loop callbacks keep 0, 1, and 2. Timers execute in scheduling order, so the three 3s come first.

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