Promises

The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

The methods promise.then(), promise.catch(), and promise.finally() are used to associate further action with a promise that becomes settled.

For example:

const myPromise = new Promise(function (resolve, reject) {
  let sampleData = [2, 4, 6, 8];
  let randomNumber = Math.floor(Math.random() * (sampleData.length + 1));
  if (sampleData[randomNumber]) {
    resolve(sampleData[randomNumber]);
  } else {
    reject('An error occured!');
  }
});

myPromise
  .then(function (e) {
    console.log(e);
  })
  .catch(function (error) {
    throw new Error(error);
  })
  .finally(function () {
    console.log('Promise completed');
  });

Methods

These methods are available on Promise.prototype

const promise1 = new Promise(function (resolve, reject) {
  resolve('Success!');
});

promise1.then(function (value) {
  console.log(value);
  // expected output: "Success!"
});

const promise1 = new Promise((resolve, reject) => {
  throw 'An error occured';
});

promise1.catch(function (error) {
  console.error(error);
});
// expected output: An error occured

References

• https://exercism.org/tracks/javascript/concepts/promises
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

1. catch
2. finally
3. then