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Intermediate Topic 19 of 51 Web Development

Promises

Objects representing eventual completion of an async operation.

How it works

A promise is an object representing the eventual result of an asynchronous operation. It is in one of three states: pending, fulfilled with a value or rejected with an error, and once settled it does not change. then registers success handlers, catch handles errors and finally runs cleanup; each returns a new promise, enabling chaining and flattening. Combinators include Promise.all, allSettled, race and any.

Step-by-step walkthrough

  1. A promise starts pending.
  2. On success it becomes fulfilled with a value.
  3. On failure it becomes rejected with an error.
  4. then handles the value and catch handles the error.
  5. Promises chain, and async/await is syntax over them.

Where it is used

APIs such as fetch, file operations, database drivers and many libraries return promises, which compose cleanly with async and await.

Common pitfall

Forgetting to return a promise inside then breaks the chain. Unhandled rejections can crash Node.js processes or go unnoticed in browsers. Creating a promise with the constructor around code that already returns a promise, the explicit construction anti-pattern, adds bugs. Promises start immediately, so they are not lazy. Always handle errors at the end of chains and use Promise.all for independent work.

Where it fits in the course

Promises is topic 19 of 51 in Web Development, rated Intermediate. It follows Asynchronous JavaScript. The next topic is Event Loop.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Web Development quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Promises?
Objects representing eventual completion of an async operation. It belongs to Web Development and is an intermediate-level topic.
Where do people use Promises?
APIs such as fetch, file operations, database drivers and many libraries return promises, which compose cleanly with async and await.
What is the most common mistake with Promises?
Forgetting to return a promise inside then breaks the chain.
What should I learn before this topic?
Start with Asynchronous JavaScript, the topic just before it in Web Development.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Web Development quiz.
Is Promises hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Promises?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Promises?
Asynchronous JavaScript, Event Loop, Browser DevTools are closely connected. The full list is in the related topics section of this page.
What should I study after Promises?
Continue with Event Loop, the next topic in Web Development.

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