Promise Chaining Interview Questions in JavaScript

Promise Chaining Interview Questions in JavaScript

Promise chaining is a top interview topic. Here are the most asked questions with code and answers.

Q1: What is the output?

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

### Q2: 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`.

### Q3: What is the output?

```js
Promise.resolve(1)
  .then((n) => Promise.resolve(n + 1))
  .then((n) => console.log(n));
``
**Answer**: `2`. If a `.then` returns a promise, the next `.then` waits for it.

### Q4: What is the output?

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

### Q5: 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`.

### Q6: 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. Fulfilled, so `.catch` is skipped, `.then` receives 1.

### Q7: What is the output?

```js
Promise.resolve(1)
  .then((n) => { throw "err"; })
  .catch((e) => "recovered")
  .then((n) => console.log(n));
``
**Answer**: `recovered`. `.catch` returns "recovered", so the next `.then` receives it.

### Q8: How do you run promises in parallel?

```js
Promise.all([fetchA(), fetchB(), fetchC()])
  .then(([a, b, c]) => console.log(a, b, c));
``
### Q9: How do you sequence promises?

Chain `.then` calls or use `async/await` with sequential `await`.

### Q10: What is the difference between .then's second arg and .catch?

`.then(onFulfilled, onRejected)` second arg only catches rejections from the original promise. `.catch` catches rejections and throws from any `.then` above it.

### The Takeaway

Promise chaining interview questions test: returning values, not returning (undefined), returning promises (wait), error handling (`.catch` catches throws), `.catch` on fulfilled (skipped), recovery after `.catch`, parallel (`Promise.all`), and `.then` second arg vs `.catch`.