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Apr 14, 2020 · Previously, we saw that we can explicitly create promises using the Promise object, whether it was by typing new Promise(() => {}), Promise.resolve, or Promise.reject. Instead of explicitly using the Promise object, we can now create asynchronous functions that implicitly return an object!
Apr 14, 2020 · Time to talk about Promises: why would you use them, how do they work “under the hood”, and how can we write them in the most modern way?
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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Aug 22, 2021 · JavaScript Promise APIs. The Promise object in JavaScript has six practical methods that serve several use cases. Promise.all. Promise.any. Promise.race. Promise.allSettled. Promise.resolve. Promise.reject. These methods take one or more promises as input and return a new promise to find the result or error.
Jun 20, 2024 · Promises are a way to handle asynchronous operations in JavaScript, allowing you to write code that is cleaner and easier to manage. They represent a value that may be available now, in the...
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Aug 15, 2023 · Why Use Promises in JavaScript? ES6 introduced promises as a native implementation. Before ES6 we were using callbacks to handle asynchronous operations. Let’s understand what callbacks are and what problem related to callbacks is solved by promises. Let's say we have a list of posts and their respective comments, like this:
