Promises are a new feature in the ES6 (ES2015) JavaScript spec that allow you to very easily deal with asynchronous code without resolving to multiple levels of callback functions. Goodbye callback hell!

Here’s how we declare a basic promise. Below you would normally call something asynchronous like an external API for example. Here we’re simulating with a setTimeout function:

let myPromise = new Promise((resolve, reject) => {
  let data;
  setTimeout(() => {
    data = "Some payload";

if (data) {
  resolve(data);
} else {
  reject();
}

Notice how a new promise is initiated with two functions as arguments, a function for its success and a function for its failure. And here’s how you would call the promise to get the payload data from it:

myPromise.then(data => {
  console.log('Received: ' + data);
}).catch(() => {
  console.log("There was an error");
});

The above will log the following at the console:

Received: Some payload

If no data was received, the following would be logged instead:

There was an error

Chaining Promises

Promises can also be chained together with multiple then functions. The return value of a then becomes the value available to the next then in the chain:

myPromise.then(data => {
  console.log('Received: ' + data);
  let moreData = "Another payload";
  return moreData;
}).then(data => {
  console.log(data);
}).catch(() => {
  console.log("There was an error");
});

The above will log the following at the console:

Received: Some payload
Another payload