by Andrea Koutifaris

Learn more about JavaScript’s promises: from zero to hero in 25 tests

A test is worth a thousand words… or was it a picture…?

I think the best way to explain JavaScript promises is through examples. What is a good, self contained, and short way to write an example? A test!

For those who have never seen a Jasmine test suit, it('...', (done) => {...}) is a test and done is a function that has to be executed when an asynchronous test is completed.

The rules here are:

  • Each test starts by asserting something in English. You have to deduce why the test code implies that the assertion of the test is true.
  • Some tests have expectations. If the test passes, the expectations are true.
  • Other tests rely on the callback done() to be called. If done() is not called, the test fails.

Each test is in this JSFiddle, so feel free to play with it while reading. Especially if you have some doubts about any of the tests, do change the test code and study what happens.

The tests

Let’s begin with Promise basics:

it('Promise executor is run SYNCHRONOUSLY', () => {  let executorRun = false;  new Promise(function executor() {    executorRun = true;  });  expect(executorRun).toBe(true);});it('you can resolve a promise', (done) => {  new Promise((resolve) => setTimeout(resolve, 1))    .then(done);});it('... or you can reject a promise', (done) => {  new Promise((resolve, reject) => setTimeout(reject, 1))    .then(undefined, done);});it('An error inside the executor, rejects the promise', (done) => {  new Promise(function executor() {    throw 'Error';  }).catch(done);});

It seems that when you call resolve() , the first then(...) callback is run. If you call reject() or an error is thrown, catch() or the second callback of then(...) is run.

Also, promise executor is run synchronously. This means that promises are a way to handle asynchronous code, not to execute tasks in asynchronous threads. Use Web Workers if you want to execute some JavaScript code outside the main thread.

Let’s see more in detail what those then(...) and catch() functions are, and what “chaining promises” means:

// Chaining promises
it('you can chain promise because .then(...) returns a promise'  , (done) => {  fetch('https://jsonplaceholder.typicode.com/posts/1')    .then(response => response.json())    .then(json => expect(json.userId).toBe(1))    .then(done);});it('you can use the fail callback of .then(success, fail) to ' +  'handle rejected promises', (done) => {  Promise.reject()    .then(function success() {      throw 'I must not be executed';    }, function fail() {      done();    });});it('... or you can use .catch() to handle rejected promises'  , (done) => {  Promise.reject()    .then(function success() {      throw 'I must not be executed';    })    .catch(done);});it('also .catch() returns a promise, allowing promise chaining'  , (done) => {  Promise.reject()    .catch(() => undefined)    .then(done);});it('you must return a rejected promise if you want to ' +  'execute the next fail callback', (done) => {  function someApiCall() {    return Promise.reject('Error');  }  someApiCall()    .catch((err) => {      console.error(err);      // Without the line below, .catch gets not called      return Promise.reject(err);    })    .catch(done);});it('... or you can throw an error if you want to ' +  'execute the next fail callback', (done) => {  function someApiCall() {    return Promise.reject('Error');  }  someApiCall()    .catch((err) => {      console.error(err);      throw err; // Without this line, .catch gets not called    })    .catch(done);});it('values returned inside .then()/.catch() callbacks ' +  'are provided to the next callback', (done) => {  Promise.resolve(1)    .then(value => value + 1)    .then(value => expect(value).toBe(2));  Promise.reject(1)    .catch(value => value + 1)    .then(value => expect(value).toBe(2));  setTimeout(() => {    done();  }, 1);});

OK, but what are Promise.resolve() and Promise.reject() ? Let’s find out!

it('you can use Promise.resolve() to wrap values or promises'  , (done) => {  function iMayReturnAPromise() {    return Math.random() >= 0.5 ? Promise.resolve() : 5;  }
  Promise.resolve(iMayReturnAPromise()).then(done);});
it('you can use Promise.resolve() to execute something just after'  , (done) => {  let arr = [];  Promise.resolve().then(() => arr.push(2));  arr.push(1);
  setTimeout(() => {    expect(arr).toEqual([1, 2]);    done();  }, 1);});
/** @seehttps://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules **/it('Promise.resolve() is normally executed before setTimeout(.., 0)'  , (done) => {  let arr = [];  setTimeout(() => arr.push('timeOut'), 0);  Promise.resolve().then(() => {    arr.push('resolve');  });
  setTimeout(() => {    expect(arr).toEqual(['resolve', 'timeOut']);    done();  }, 1);});
it('you can create rejected promises', (done) => {  Promise.reject('reason').catch(done);});
it('pay attention to "Uncaught (in promise) ..."', () => {  Promise.reject('The error');  // Outputs in the console Uncaught (in promise) The error});

Chaining promises vs. creating new ones

Although new Promise(...) is a way to create a promise, you should avoid using it. Most of the time, functions/libraries return a promise, so you should chain promises and not create new ones:

it("Don't use new Promise(...), prefer chaining", (done) => {  const url = 'https://jsonplaceholder.typicode.com/posts/1';  function badlyDesignedCustomFetch() {    return new Promise((resolve, reject) => {      fetch(url).then((response) => {        if (response.ok) {          resolve(response);        } else {          reject('Fetch failed');        }      });    });  }  function wellDesignedCustomFetch() {    return fetch(url).then((response) =>; {      if (!response.ok) {        return Promise.reject('Fetch failed');      }      return (response);    });  }  Promise.all([    badlyDesignedCustomFetch(),    wellDesignedCustomFetch()  ]).then(done);});

But, when should you use new Promise(...) ? When you want to move from a callback interface to a Promise one. See below:

function imgOnLoad(img) {  return new Promise((resolve, reject) => {    img.onload = resolve;    img.onerror = reject;  });}

Parallel execution

Promise chaining is nice, but what about executing asynchronous operations in parallel? Below is all you need to know:

// Parallel execution of promises
it('you can use Promise.all([...]) to execute promises in parallel'  , (done) => {  const url = 'https://jsonplaceholder.typicode.com/posts';  const p1 = fetch(`${url}/1`);  const p2 = fetch(`${url}/2`);  Promise.all([p1, p2])    .then(([res1, res2]) => {      return Promise.all([res1.json(), res2.json()])    })    .then(([post1, post2]) => {      expect(post1.id).toBe(1);      expect(post2.id).toBe(2);    })    .then(done);});it('Promise.all([...]) will fail if any of the promises fails'  , (done) => {  const p1 = Promise.resolve(1);  const p2 = Promise.reject('Error');  Promise.all([p1, p2])    .then(() => {      fail('I will not be executed')    })    .catch(done);});it("if you don't want Promise.all() to fail, wrap the promises " +  "in a promise that will not fail", (done) => {  function iMayFail(val) {    return Math.random() >= 0.5 ?      Promise.resolve(val) :      Promise.reject(val);  }  function promiseOr(p, value) {    return p.then(res => res, () => value);  }  const p1 = iMayFail(10);  const p2 = iMayFail(9);  Promise.all([promiseOr(p1, null), promiseOr(p2, null)])    .then(([val1, val2]) => {      expect(val1 === 10 || val1 === null).toBe(true);      expect(val2 === 9 || val2 === null).toBe(true);    })    .catch(() => {      fail('I will not be executed')    })    .then(done);});it('Promise.race([...]) will resolve as soon as ' +  'one of the promises resolves o rejects', (done) => {  const timeout =    new Promise((resolve, reject) => setTimeout(reject, 100));  const data =    fetch('https://jsonplaceholder.typicode.com/posts/1');  Promise.race([data, timeout])    .then(() => console.log('Fetch OK'))    .catch(() => console.log('Fetch timeout'))    .then(done);});

Syntax

Promise syntax is a bit complex compared to the typical syntax of synchronous code. It is true that by chaining promises, the code keeps good readability, but it could be better. The new await/async syntax makes using promises as easy as writing synchronous code.

// New await/async syntax
it('you can use the new await/async syntax', async () => {  function timeout(ms) {    return new Promise((resolve) => setTimeout(resolve, ms));  }  const start = Date.now();  const delay = 200;  await timeout(delay + 2); // Just some ms tolerance  expect(Date.now() - start).toBeGreaterThanOrEqual(delay);});it('an async function returns a promise', (done) => {  async function iAmAsync() {    return 1;  }  iAmAsync()    .then((val) => expect(val).toBe(1))    .then(done);});it('await just awaits a promise resolution', async (done) => {  await Promise.resolve();  done();});it('await will throw an error if the promise fail', async(done) => {  try {    await Promise.reject();    fail('I will not be executed');  } catch (err) {    done();  }});

Synchronous functions

On last consideration: when you design a function, you have to decide whether it is synchronous or not. Don’t return a promise just because “you never know”. Use “normal” synchronous functions when possible.

All the tests are here, on JSFiddle.

That’s all! Hope you enjoyed this article.