Mihai Serban

Cluj-Napoca, Romania 🇷🇴

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ES6 cheatsheet  —  Promises

Serban Mihai / 20 October 2018

~2 min read

ES6 Promises

Promises are one of the most exciting additions to JavaScript ES6. Promises are a pattern that greatly simplifies asynchronous programming by making the code look synchronous and avoid problems associated with callbacks.

Prior to ES6, we used bluebird or Q. Now we have Promises natively.

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

The resolve and reject are functions themselves and are used to send back values to the promise object.

const myPromise = new Promise((resolve, reject) => {  
    if (Math.random() * 100 <= 90) {  
        resolve('Hello, Promises!');  
    }  
    reject(new Error('In 10% of the cases, I fail. Miserably.'));  
});  

myPromise.then((resolvedValue) => {  
    console.log(resolvedValue); //Hello, Promises!  
}, (error) => {  
    console.log(error); //In 10% of the cases, I fail. Miserably.  
});

Chaining Promises:

Promises allow us to turn our horizontal code (callback hell):

func1(function (value1) {  
    func2(value1, function (value2) {  
        func3(value2, function (value3) {  
          // Do something with value 3  
        });  
    });  
});

Into vertical code like so:

func1(value1)  
    .then(func2)  
    .then(func3, value3 => {  
        // Do something with value 3  
    });

Parallelize Promises:

We can use Promise.all() to handle an array of asynchronous operations.

let urls = [  
  '/api/commits',  
  '/api/issues/opened',  
  '/api/issues/assigned',  
  '/api/issues/completed',  
  '/api/issues/comments',  
  '/api/pullrequests'  
];  

let promises = urls.map((url) => {  
  return new Promise((resolve, reject) => {  
    $.ajax({ url: url })  
      .done((data) => {  
        resolve(data);  
      });  
  });  
});  

Promise.all(promises)  
  .then((results) => {  
    // Do something with results of all our promises  
 });

You can find a more complete ES6 cheetsheet on my Github page.

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