setTimeout Interview Questions: Trust Issues in JavaScript

setTimeout Interview Questions: Trust Issues in JavaScript

setTimeout timing is a favorite interview topic. Here are the most asked questions.

Q1: Does setTimeout(cb, 1000) guarantee exact 1000ms?

No. It guarantees a minimum of 1000ms. The actual delay depends on the call stack, microtasks, background tab throttling, and nested timer clamping.

Q2: What is the output?

setTimeout(() => console.log("timer"), 0);
for (let i = 0; i < 1e9; i++) {}

Answer: "timer" logs after 5 seconds. The callback was queued; the event loop could not run it while the loop was blocking the stack.

Q3: What is the minimum delay for nested setTimeout?

4ms after 5 nested levels (HTML spec).

Q4: Does setTimeout work normally in background tabs?

No. Browsers throttle timers in background tabs to ~1000ms minimum.

Q5: What is the output?

console.log("start");
setTimeout(() => console.log("timeout"), 0);
Promise.resolve().then(() => console.log("promise"));
console.log("end");

Answer: start, end, promise, timeout. Microtasks before macrotasks.

Q6: Can setTimeout callbacks run before the delay?

No. The callback never runs before the specified delay. It may run later, but never earlier.

Q7: What happens if you pass a negative delay?

The delay is treated as 0. The callback goes to the macrotask queue.

Q8: How do you cancel a setTimeout?

const id = setTimeout(cb, 1000);
clearTimeout(id);

The Takeaway

setTimeout interview questions test: the minimum-delay guarantee, blocking the stack, FIFO ordering, nested timer clamping (4ms), background tab throttling, microtasks before macrotasks, negative delay (treated as 0), and clearTimeout.