Event Loop Interview Questions in JavaScript

Event Loop Interview Questions in JavaScript

The event loop is one of the most asked interview topics. Here are the common questions with answers and code.

Q1: What is the output?

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
``
**Answer**: `1, 4, 3, 2`. Sync first, then microtasks, then macrotasks.

### Q2: What is the output?

```js
setTimeout(() => console.log("a"), 0);
Promise.resolve().then(() => console.log("b"));
console.log("c");
``
**Answer**: `c, b, a`. Sync, microtask, macrotask.

### Q3: What is the output?

```js
Promise.resolve().then(() => console.log("1")).then(() => console.log("2"));
Promise.resolve().then(() => console.log("3")).then(() => console.log("4"));
``
**Answer**: `1, 3, 2, 4`. Microtasks are FIFO. The first .then of each promise runs before the second .then.

### Q4: 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`.

### Q5: What is the output?

```js
async function foo() {
  console.log("1");
  await Promise.resolve();
  console.log("2");
}
foo();
console.log("3");
``
**Answer**: `1, 3, 2`. `await` pauses `foo` and puts the rest in a microtask. `3` logs first (sync), then `2` (microtask).

### Q6: What is the output?

```js
console.log("start");
setTimeout(() => console.log("timeout"), 0);
Promise.resolve().then(() => {
  console.log("promise1");
  Promise.resolve().then(() => console.log("promise2"));
});
console.log("end");
``
**Answer**: `start, end, promise1, promise2, timeout`. Nested microtasks run before the macrotask.

### Q7: Can microtasks block rendering?

**Answer**: Yes. The event loop drains all microtasks before rendering. If microtasks take too long, the paint is delayed. This is why you should keep microtasks short.

### Q8: Can microtasks starve macrotasks?

**Answer**: Yes. If a microtask keeps adding more microtasks, the event loop never reaches the macrotask queue. setTimeout callbacks never run.

### Q9: Is the event loop part of the JS engine?

**Answer**: No. The event loop is part of the host environment (browser or Node.js). The JS engine (V8) just has the call stack and heap. The event loop, Web APIs, and queues are provided by the host.

### Q10: What is `process.nextTick` in Node.js?

**Answer**: A Node.js-specific microtask that runs before promise callbacks. It has higher priority than Promise.then. Use it carefully; it can starve promises and the event loop.

### The Takeaway

Event loop interview questions test: sync vs microtask vs macrotask ordering, nested microtasks, the loop closure bug, async/await timing, microtask starvation, and rendering. Remember: sync → all microtasks → one macrotask → microtasks → repeat.