Understanding Promises in JavaScript: A Comprehensive Guide

Barış Çimen
6 min readMar 25, 2023

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JavaScript is an incredibly versatile and powerful programming language, and one of its most essential features is the ability to work with asynchronous operations. When dealing with asynchronous code, Promises are an indispensable tool that can make your code more efficient, maintainable, and easy to read.

A promise is an object that represents a value that may not be available yet but will be resolved in the future. A promise has three states: pending, fulfilled, or rejected.

  • The pending state means that the promise is neither fulfilled nor rejected yet.
  • The fulfilled state means that the promise has been resolved and that it has value.
  • The rejected state means that the promise has been resolved with an error.

The Key Benefit of using Promise

Promise allows you to work with asynchronous operations in a way that is both simple and intuitive. Rather than using callbacks, which can quickly become difficult to manage and read, promises enable you to handle the results of an asynchronous operation in a way that is more structured and easy to understand.

How to use Promise?

The Promise constructor takes a single argument, which is a function that defines the asynchronous operation that you want to perform. This function takes two arguments: resolve and reject. When the asynchronous operation is complete, you call one of these functions to indicate whether the promise has been fulfilled or rejected.

Here’s an example of a simple promise that resolves after a specified amount of time:

const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('Operation completed successfully!');
}, 1000);
});

promise.then(result => console.log(result));

// Expected output:
// Operation completed successfully!

In this example, the promise takes a function that sets a timeout of one second before resolving the promise with a message. To handle the result of this promise, you can attach a callback to the then() method, which will be called with the resolved value when the promise is fulfilled.

Promise also provides a way to handle errors that may occur during an asynchronous operation. If you call the reject() function inside the promise function, the promise will be rejected, and you can handle the error with the catch method:

const promise = new Promise((resolve, reject) => {
setTimeout(() => {
reject(new Error('Operation failed!'));
}, 1000);
});

promise.catch(error => {
console.error(error.message);
});

// Expected output:
// Operation failed!

In this example, the promise is rejected with an error after one second and the catch() method logs the error message to the console.

You can also use the finally() method of the Promise object to execute a piece of code after the promise has been resolved or rejected, regardless of the outcome. The finally() method takes a single function parameter that will be executed after the promise is settled.

Here’s an example of using the finally() method to execute some cleanup code after a promise is resolved or rejected:

const promise = new Promise((resolve, reject) => {
setTimeout(() => {
reject(new Error('Operation failed!'));
}, 1000);
});

promise.catch(error => {
console.error(error.message);
}).finally(() => {
console.log('Cleanup code executed!');
});

// Expected output:
// Operation failed!
// Cleanup code executed!

In this example, the promise is rejected with an error after one second, and the catch() method logs the error message to the console. The finally() method is then called to execute some cleanup code, which logs a message to the console.

Other Promise Methods

In addition to the then(), catch(), and finally() methods, Promise provides several other methods that can be used to compose and manipulate promises.

▹ Promise.all()

This static method takes an array of promises and returns a new promise that resolves with an array of all the resolved values in the same order as the original array. If any of the promises in the array rejects, the new promise immediately rejects with the error from the first rejected promise. For example:

const promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 500, 'success1');
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(resolve, 1000, 'success2');
});
const promise3 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'success3');
});

Promise.all([promise1, promise2, promise3]).then((values) => {
console.log(values);
});

// Expected output:
// Array ["success1", "success2", "success3"]

In this example, the Promise.all() method takes an array of promises and waits for all of them to be fulfilled before logging the message to the console.

▹ Promise.allSettled()

This static method takes an array of promises and returns a new promise that resolves with an array of objects representing the final state (fulfilled or rejected) of each promise in the array, including the value or reason. For example:

const promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 500, 'Promise 1 resolved');
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(reject, 1000, 'Promise 2 rejected');
});
const promise3 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'Promise 3 resolved');
});

Promise.allSettled([promise1, promise2, promise3])
.then(results => console.log(results));

// Expected output:
// Array [Object { status: "fulfilled", value: "Promise 1 resolved" }, Object { status: "rejected", reason: "Promise 2 rejected" }, Object { status: "fulfilled", value: "Promise 3 resolved" }]

In this example, the new promise returned by Promise.allSettled() resolves with an array of objects representing the final state of each promise in the array, including the value or reason. The first and third promises are fulfilled, whereas the second promise is rejected before logging the message to the console.

▹ Promise.race()

This static method takes an array of promises and returns a new promise that resolves or rejects as soon as one of the promises in the array resolves or rejects. For example:

const promise1 = new Promise(resolve => setTimeout(() => resolve('Promise 1 resolved'), 1000));
const promise2 = new Promise((resolve, reject) => setTimeout(() => reject('Promise 2 rejected'), 500));

Promise.race([promise1, promise2])
.then(result => console.log(result))
.catch(error => console.error(error));

// Expected output:
// Promise 2 rejected

In this example, the Promise.race() method takes an array of two promises, one that resolves after one second and one that rejects after 500ms. Since the second promise rejects before the first one is resolved, the new promise is returned by Promise.race() rejects with the error message.

▹ Promise.resolve()

This static method returns a new promise that is already resolved with a specified value. For example:

const promise = Promise.resolve('Promise resolved!');

promise.then(result => console.log(result));

// Expected output:
// Promise resolved!

In this example, the Promise.resolve() method returns a new promise that is immediately resolved with the value Promise resolved!.

▹ Promise.reject()

This static method returns a new promise that is already rejected with a specified error. For example:

const promise = Promise.reject(new Error('Promise rejected!'));

promise.catch(error => console.error(error.message));

// Expected output:
// Promise rejected!

In this example, the Promise.reject() method returns a new promise that is immediately rejected with the error Promise rejected!. The error can be caught using the catch method.

Support in Popular Browsers

Promises were introduced in the ECMAScript 6 (ES6) specification, which was published in June 2015. Since then, most modern browsers have implemented support for Promises. Here’s a breakdown of when Promises were first supported by some of the most popular browsers:

  • Chrome 32 (January 2014): Promises were initially supported in Chrome as an experimental feature behind a flag. They were enabled by default in Chrome 33 (February 2014).
  • Firefox 29 (April 2014): Promises were initially supported in Firefox as an experimental feature behind a flag. They were enabled by default in Firefox 32 (September 2014).
  • Safari 8 (June 2014): Promises were initially supported in Safari as an experimental feature behind a flag. They were enabled by default in Safari 8.1 (April 2015).
  • Edge 12 (July 2015): Promises were supported in the initial release of Microsoft Edge.

Additionally, if you need to support older browsers that don’t have built-in Promise support, you can use a polyfill, which is a piece of JavaScript code that provides Promise functionality in older browsers. There are several popular Promise polyfills available, such as es6-promise and bluebird.

Conclusion

Promises are a powerful and essential tool in modern JavaScript programming. They enable you to write asynchronous code in a way that is simple, structured, and easy to understand. By mastering promises, you can create more efficient and maintainable code that is better suited to the demands of modern web development.

References

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