setTimeout Interview Questions in JavaScript

setTimeout Interview Questions in JavaScript

setTimeout is a common interview topic, especially combined with closures and the event loop. Here are the most asked questions.

Q1: What is the output?

console.log("1");
setTimeout(() => console.log("2"), 0);
console.log("3");
``
**Answer**: `1, 3, 2`. The timer callback goes to the macrotask queue and runs after the synchronous code.

### Q2: What is the output?

```js
setTimeout(() => console.log("timer"), 0);
Promise.resolve().then(() => console.log("promise"));
``
**Answer**: `promise, timer`. Microtasks (promises) run before macrotasks (timers).

### Q3: What is the output?

```js
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}
``
**Answer**: `3, 3, 3`. `var` is function-scoped; all callbacks share one `i`.

### Q4: What is the output?

```js
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}
``
**Answer**: `0, 1, 2`. `let` creates a fresh binding per iteration.

### Q5: What is the output?

```js
for (var i = 0; i < 3; i++) {
  setTimeout((j) => console.log(j), 0, i);
}
``
**Answer**: `0, 1, 2`. The extra argument `i` is passed by value as `j`.

### Q6: What is the output?

```js
setTimeout(() => console.log("a"), 0);
setTimeout(() => console.log("b"), 0);
setTimeout(() => console.log("c"), 0);
``
**Answer**: `a, b, c`. Timers run in FIFO order (first in, first out) for the same delay.

### Q7: What is the output?

```js
function foo() {
  setTimeout(() => console.log("inner"), 0);
  console.log("outer");
}
foo();
``
**Answer**: `outer, inner`. The timer is queued; `"outer"` logs first.

### Q8: Does `setTimeout(cb, 0)` run `cb` immediately?

**Answer**: No. It goes to the macrotask queue. The event loop runs it after the stack is empty and microtasks are done.

### Q9: Can `setTimeout` be early?

**Answer**: No. The callback never runs before the delay. It may run later (if the stack is busy or microtasks are queued), but never earlier.

### Q10: How do you cancel a `setTimeout`?

```js
const id = setTimeout(() => console.log("hi"), 1000);
clearTimeout(id); // cancels it
``
### The Takeaway

`setTimeout` interview questions test: synchronous vs async ordering, microtasks vs macrotasks, the loop closure bug (`var` vs `let`), extra arguments, FIFO ordering, and `clearTimeout`. Remember: microtasks run before macrotasks, `var` shares one `i`, `let` creates fresh bindings, and `setTimeout(cb, 0)` does not run immediately.