What is the settle once?

Only the first resolve/reject call counts.

What is the then callback return?

Becomes the next promise's value; none means undefined.

What is the finally callback?

No argument; return value ignored unless it throws or rejects.

What is the promise.all?

Input-order array; rejects on first rejection.

What is the promise.allSettled?

Never rejects; {status, value/reason} per input.

What is the promise.race?

Mirrors the first input to settle.

What is the empty input array?

all fulfills with []; any rejects with AggregateError; race stays pending forever.

What is the promise.any all rejected?

AggregateError; errors in input order.

Asynchronous Programming13% of the exam

Promises, async and await

How a promise settles once, how values and errors travel through then, catch and finally, which of Promise.all, allSettled, race and any fits a requirement, how to wrap a callback API in a promise, and how async functions and await behave, including sequential versus parallel awaits, missing await or return bugs, and top-level await.

Official exam objective

Given a scenario, apply asynchronous programming concepts.

~7 min read · 21 practice questions · updated Oct 16, 2026

What the exam expects

Predict what a promise chain logs: settle-once, values returned (or not) from then, errors skipping to catch, recovery after catch, and the transparent finally.
Pick Promise.all, allSettled, race or any from a requirement, and predict result order and failure behavior.
Wrap a callback API in new Promise and flatten nested callbacks with async/await.
Use async functions correctly: they always return a promise, await runs steps sequentially unless you start them first, and a missing await or return loses ordering and errors.
Key terms: pending / fulfilled / rejected (a promise's states; settled means not pending); executor (the function passed to new Promise that calls resolve or reject); chain (each then returns a new promise); floating promise (one started but neither returned nor awaited); top-level await (await outside any function, valid only in modules).

Key facts

Fact	Detail	Source
Settle once	Only the first `resolve`/`reject` call counts	Promise() constructor (opens in a new tab)
`then` callback return	Becomes the next promise's value; none means `undefined`	then() (opens in a new tab)
`finally` callback	No argument; return value ignored unless it throws or rejects	finally() (opens in a new tab)
`Promise.all`	Input-order array; rejects on first rejection	Promise.all() (opens in a new tab)
`Promise.allSettled`	Never rejects; `{status, value/reason}` per input	Promise.allSettled() (opens in a new tab)
`Promise.race`	Mirrors the first input to settle	Promise.race() (opens in a new tab)
Empty input array	`all` fulfills with `[]`; `any` rejects with `AggregateError`; `race` stays pending forever	Promise.all() (opens in a new tab)Promise.any() (opens in a new tab)Promise.race() (opens in a new tab)
`Promise.any` all rejected	`AggregateError`; `errors` in input order	Promise.any() (opens in a new tab)
Async function result	Always a promise; uncaught throw rejects it	async function (opens in a new tab)
Top-level `await`	Only in modules; elsewhere a `SyntaxError`	await (opens in a new tab)
`forEach` with async callback	Does not wait for the promises	forEach() (opens in a new tab)
Response body	Readable once; `clone()` before reading to reuse	Using the Fetch API (opens in a new tab)

Settle once

Detail: Only the first resolve/reject call counts
Source: Promise() constructor (opens in a new tab)

then callback return

Detail: Becomes the next promise's value; none means undefined
Source: then() (opens in a new tab)

finally callback

Detail: No argument; return value ignored unless it throws or rejects
Source: finally() (opens in a new tab)

Promise.all

Detail: Input-order array; rejects on first rejection
Source: Promise.all() (opens in a new tab)

Promise.allSettled

Detail: Never rejects; {status, value/reason} per input
Source: Promise.allSettled() (opens in a new tab)

Promise.race

Detail: Mirrors the first input to settle
Source: Promise.race() (opens in a new tab)

Empty input array

Detail: all fulfills with []; any rejects with AggregateError; race stays pending forever
Source: Promise.all() (opens in a new tab)Promise.any() (opens in a new tab)Promise.race() (opens in a new tab)

Promise.any all rejected

Detail: AggregateError; errors in input order
Source: Promise.any() (opens in a new tab)

Async function result

Detail: Always a promise; uncaught throw rejects it
Source: async function (opens in a new tab)

Top-level await

Detail: Only in modules; elsewhere a SyntaxError
Source: await (opens in a new tab)

forEach with async callback

Detail: Does not wait for the promises
Source: forEach() (opens in a new tab)

Response body

Detail: Readable once; clone() before reading to reuse
Source: Using the Fetch API (opens in a new tab)

Sources for this section (11)

Check yourself

Why does `100 - getDiscount()` log `NaN` when `getDiscount` is an async function returning 10?
When should you choose `Promise.race` over `Promise.any`?
Why does a chain continue normally after a `catch` that returns a value?
In what order does `Promise.all` return results from inputs that finish at different times?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

Keep reading with the JavaScript Developer pack

Core concepts
Exam traps
Worked scenario

The pack includes:

All 24 lessons of the JavaScript Developer guide, with reading progress
Interactive decision walks and step-by-step sequence steppers
Inline practice questions and one-click topic practice (21 questions for this lesson)
An official source cited for every claim
An AI tutor that answers from the guide
Unlimited timed practice exams

Get the pack Already own it? Sign in

What the exam expects

Key facts

Check yourself

Keep reading with the JavaScript Developer pack

Practice for the JavaScript Developer exam