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Jun 13, 2023 · This article is an in-depth guide to promises in JavaScript. You are going to learn why JavaScript has promises, what a promise is, and how to work with it. You are also going to learn how to use async/await—a feature derived from promises—and what a job queue is. Here are the topics we will cover: Why should you care about promises?
- Overview
- Chaining
- Error handling
- Composition
- Creating a Promise around an old callback API
- Timing
- See also
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A Promise is an object representing the eventual completion or failure of an asynchronous operation. Since most people are consumers of already-created promises, this guide will explain consumption of returned promises before explaining how to create them.
Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function. Imagine a function, createAudioFileAsync(), which asynchronously generates a sound file given a configuration record and two callback functions: one called if the audio file is successfully created, and the other called if an error occurs.
Here's some code that uses createAudioFileAsync():
If createAudioFileAsync() were rewritten to return a promise, you would attach your callbacks to it instead:
This convention has several advantages. We will explore each one.
A common need is to execute two or more asynchronous operations back to back, where each subsequent operation starts when the previous operation succeeds, with the result from the previous step. In the old days, doing several asynchronous operations in a row would lead to the classic callback pyramid of doom:
With promises, we accomplish this by creating a promise chain. The API design of promises makes this great, because callbacks are attached to the returned promise object, instead of being passed into a function.
Here's the magic: the then() function returns a new promise, different from the original:
This second promise (promise2) represents the completion not just of doSomething(), but also of the successCallback or failureCallback you passed in — which can be other asynchronous functions returning a promise. When that's the case, any callbacks added to promise2 get queued behind the promise returned by either successCallback or failureCallback.
Note: If you want a working example to play with, you can use the following template to create any function returning a promise:
The implementation is discussed in the Creating a Promise around an old callback API section below.
You might recall seeing failureCallback three times in the pyramid of doom earlier, compared to only once at the end of the promise chain:
If there's an exception, the browser will look down the chain for .catch() handlers or onRejected. This is very much modeled after how synchronous code works:
This symmetry with asynchronous code culminates in the async/await syntax:
Promises solve a fundamental flaw with the callback pyramid of doom, by catching all errors, even thrown exceptions and programming errors. This is essential for functional composition of asynchronous operations. All errors are now handled by the catch() method at the end of the chain, and you should almost never need to use try/catch without using async/await.
There are four composition tools for running asynchronous operations concurrently: Promise.all(), Promise.allSettled(), Promise.any(), and Promise.race().
We can start operations at the same time and wait for them all to finish like this:
If one of the promises in the array rejects, Promise.all() immediately rejects the returned promise and aborts the other operations. This may cause unexpected state or behavior. Promise.allSettled() is another composition tool that ensures all operations are complete before resolving.
These methods all run promises concurrently — a sequence of promises are started simultaneously and do not wait for each other. Sequential composition is possible using some clever JavaScript:
In this example, we reduce an array of asynchronous functions down to a promise chain. The code above is equivalent to:
This can be made into a reusable compose function, which is common in functional programming:
A Promise can be created from scratch using its constructor. This should be needed only to wrap old APIs.
In an ideal world, all asynchronous functions would already return promises. Unfortunately, some APIs still expect success and/or failure callbacks to be passed in the old way. The most obvious example is the setTimeout() function:
Mixing old-style callbacks and promises is problematic. If saySomething() fails or contains a programming error, nothing catches it. This is intrinsic to the design of setTimeout.
Luckily we can wrap setTimeout in a promise. The best practice is to wrap the callback-accepting functions at the lowest possible level, and then never call them directly again:
Guarantees
In the callback-based API, when and how the callback gets called depends on the API implementor. For example, the callback may be called synchronously or asynchronously: The above design is strongly discouraged because it leads to the so-called "state of Zalgo". In the context of designing asynchronous APIs, this means a callback is called synchronously in some cases but asynchronously in other cases, creating ambiguity for the caller. For further background, see the article Designing APIs for Asynchrony, where the term was first formally presented. This API design makes side effects hard to analyze: On the other hand, promises are a form of inversion of control — the API implementor does not control when the callback gets called. Instead, the job of maintaining the callback queue and deciding when to call the callbacks is delegated to the promise implementation, and both the API user and API developer automatically gets strong semantic guarantees, including: •Callbacks added with then() will never be invoked before the completion of the current run of the JavaScript event loop. •These callbacks will be invoked even if they were added after the success or failure of the asynchronous operation that the promise represents. •Multiple callbacks may be added by calling then() several times. They will be invoked one after another, in the order in which they were inserted.
Task queues vs. microtasks
Promise callbacks are handled as a microtask whereas setTimeout() callbacks are handled as task queues. The code above will output: For more details, refer to Tasks vs. microtasks.
When promises and tasks collide
If you run into situations in which you have promises and tasks (such as events or callbacks) which are firing in unpredictable orders, it's possible you may benefit from using a microtask to check status or balance out your promises when promises are created conditionally. If you think microtasks may help solve this problem, see the microtask guide to learn more about how to use queueMicrotask() to enqueue a function as a microtask.
•Promise
•async function
•await
•Promises/A+ specification
•We have a problem with promises on pouchdb.com (2015)
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Jul 25, 2024 · Promises are the foundation of asynchronous programming in modern JavaScript. A promise is an object returned by an asynchronous function, which represents the current state of the operation.
Jun 23, 2024 · A promise is a special JavaScript object that links the “producing code” and the “consuming code” together. In terms of our analogy: this is the “subscription list”. The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes that result available to all of the subscribed code when it’s ready.
Here is how to use a Promise: myPromise.then(. function(value) { /* code if successful */ }, function(error) { /* code if some error */ } ); Promise.then () takes two arguments, a callback for success and another for failure. Both are optional, so you can add a callback for success or failure only.
Oct 7, 2024 · The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. To learn about the way promises work and how you can use them, we advise you to read Using promises first.
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Feb 13, 2024 · Joe Attardi. Many operations, such as network requests, are asynchronous in nature. One of the most useful and powerful tools for working with asynchronous code is the Promise. In this handbook, you'll learn all about JavaScript Promises and how to use them. Table of Contents. What is a Promise? Comparing Promises to Other Async Patterns.
