Promise Interview Questions in JavaScript

Promise Interview Questions in JavaScript

Promises are a top interview topic. Here are the most asked questions with answers and code.

Q1: What is a promise?

An object representing the eventual result of an async operation. Three states: pending, fulfilled, rejected. Settles once.

Q2: What is the output?

console.log("1");
Promise.resolve().then(() => console.log("2"));
console.log("3");
``
**Answer**: `1, 3, 2`. `.then` is a microtask; it runs after the synchronous code.

### Q3: What is the output?

```js
Promise.resolve(1)
  .then((n) => n + 1)
  .then((n) => console.log(n));
``
**Answer**: `2`. Each `.then` passes its return value to the next.

### Q4: What is the output?

```js
Promise.resolve(1)
  .then((n) => { n + 1; })
  .then((n) => console.log(n));
``
**Answer**: `undefined`. The first `.then` has a block body with no `return`.

### Q5: What is the output?

```js
Promise.reject(new Error("oops"))
  .then((v) => console.log("then", v))
  .catch((err) => console.log("catch", err.message));
``
**Answer**: `catch oops`. The `.then` is skipped (promise rejected); `.catch` runs.

### Q6: What is the output?

```js
Promise.resolve(1)
  .then((n) => { throw new Error("oops"); })
  .then((n) => console.log("skipped"))
  .catch((err) => console.log(err.message));
``
**Answer**: `oops`. The throw skips the next `.then` and goes to `.catch`.

### Q7: What is the output?

```js
Promise.resolve(1)
  .catch((err) => console.log("catch"))
  .then((n) => console.log("then", n));
``
**Answer**: `then 1`. `.catch` only runs on rejection. Since the promise is fulfilled, `.catch` is skipped and `.then` receives 1.

### Q8: What is the output?

```js
Promise.all([Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)])
  .then((results) => console.log(results));
``
**Answer**: `[1, 2, 3]`. `Promise.all` waits for all and returns results in order.

### Q9: What is the output?

```js
Promise.all([Promise.resolve(1), Promise.reject("err"), Promise.resolve(3)])
  .then((results) => console.log(results))
  .catch((err) => console.log("catch", err));
``
**Answer**: `catch err`. `Promise.all` rejects if any promise rejects.

### Q10: What is the difference between Promise.all and Promise.allSettled?

`Promise.all` rejects if any promise rejects (fast fail). `Promise.allSettled` waits for all and returns `[{ status, value/reason }]` for each.

### The Takeaway

Promise interview questions test: the definition, microtask ordering (promises run after sync), chaining (return values), error handling (`.catch` catches throws and rejections), `.catch` on a fulfilled promise (skipped), and `Promise.all` vs `Promise.allSettled`.