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JS promise

last modified October 18, 2023

In this article we show how to work with promises in JavaScript.

A represents a potential value, or error, that will be available at some time in the future.

A Promise can have one of the states:

We work with promises using either callbacks or async/await keywords.

Promises are called futures in some programming languages.

JS Promise.resolve

The Promise.resolve method returns a Promise object that is resolved with a given value.

main.js
let promise = new Promise(resolve => {

    setTimeout(() => resolve(2), 2000);
});

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

console.log('finished');

We create a promise which resolves after two seconds with an integer value. The then function attaches callbacks for the resolution and/or rejection of the promise. 

$ node main.js
finished
2

In the next example, we use the async/await keywords.

main.js
async function doWork() {

    let res = await promise;
    console.log(res);
}

let promise = new Promise(resolve => {

    setTimeout(() => resolve(2), 2000);
});

doWork();

console.log('finished');

The await keyword is used to wait for a Promise. It can only be used inside an async function.

JS promise error

The Promise.reject method returns a Promise object that is rejected with a given reason.

main.js
let promise = new Promise((resolve, reject) => {

    let b = Math.random() < 0.3;

    if (b) {
        resolve(10);
    } else {
        reject("promise error");
    }
});

promise.then(val => console.log(val)).catch(err => {
    console.log(`${err}`)
});

console.log('finished');

In the example, we simulate an error with Math.random. We catch the error with catch method. 

$ node main.js
finished
promise error
$ node main.js
finished
promise error
$ node main.js
finished
10

JS chaining promises

It is possible to execute multiple asynchronous operations with chaining.

main.js
let p = new Promise((resolve) => {
    resolve(1);
});

let r = p.then(val => val + 2).then(val => val + 3)
    .then(val => val + 4).then(val => console.log(val));

console.log('finished');

The example adds four integers via chaining of promises. 

$ node chain.js
finished
10

JS Promise.all

The Promise.all method takes an iterable of promises as a parameter and returns a single promise that resolves to an array of the results of the given promises.

This returned promise resolves when all promises have resolved. It rejects immediately upon any of the input promises rejecting with error.

main.js
const p1 = new Promise((resolve) => setTimeout(resolve, 100, 100));
const p2 = new Promise((resolve) => setTimeout(resolve, 300, 200));
const p3 = new Promise((resolve) => setTimeout(resolve, 500, 300));

const promises = [p1, p2, p3];

Promise.all(promises).then((data) =>
    console.log(data.reduce((total, next) => total + next)));

console.log('finished');

In the example, we wait for all promises to finish and in the end, we calculate the sum of returned values. 

$ node all.js
finished
600

JS multiple requests with axios

In the next example, we use the axois library to execute multiple get requests. Axios is a promise-based HTTP client for Node and the browser. 

$ npm i axios

We install the Axios library.

main.js
const axios = require('axios');

async function makeRequests(urls) {

    const fetchUrl = (url) => axios.get(url);
    const promises = urls.map(fetchUrl);

    let responses = await Promise.all(promises);

    responses.forEach(resp => {

        let msg = `${resp.config.url} -> ${resp.headers.server}: ${resp.status}`;
        console.log(msg);
    });
}

let urls = [
    'http://webcode.me',
    'https://example.com',
    'http://httpbin.org',
    'https://clojure.org',
    'https://fsharp.org',
    'https://symfony.com',
    'https://www.perl.org',
    'https://www.php.net',
    'https://www.python.org',
    'https://code.visualstudio.com',
    'https://github.com'
];

makeRequests(urls);

We wait for all requests to finish with Promise.all. After completion, we go through the array of responses and server name and response status. 

$ node main.js
http://webcode.me -> nginx/1.6.2: 200
https://example.com -> ECS (dcb/7ECA): 200
http://httpbin.org -> gunicorn/19.9.0: 200
https://clojure.org -> AmazonS3: 200
https://fsharp.org -> GitHub.com: 200
https://symfony.com -> cloudflare: 200
https://www.perl.org -> Combust/Plack (Perl): 200
https://www.php.net -> myracloud: 200
https://www.python.org -> nginx: 200
https://code.visualstudio.com -> Microsoft-IIS/10.0: 200
https://github.com -> GitHub.com: 200

JS create PDF with Puppeteer

In the following example, we genearate a PDF file from a webpage using the Puppeteer library. 

$ npm i puppeteer

We install Puppeteer.

main.js
const puppeteer = require('puppeteer');

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.goto('http://webcode.me');
  await page.pdf({ path: 'webcode.pdf', format: 'a5' });

  await browser.close();
})();

The whole task consists of several asynchronous operations, which are handled with async/await keywords.

Source

JS Promise - language reference

In this article we have worked with promises in JavaScript.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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