Promises

Promises, in one form or another, have been available in JavaScript for a while. All jQuery users are familiar with the idea. A promise is a reference to a variable that is generated asynchronously and may become available in the future.

The ES5 way of doing things, if you weren't already using some sort of third-party promise library or jQuery's deferred's, was to accept a callback function to an asynchronous method and run the callback upon successful completion, as shown in the following code:

function updateUser(user, settings, onComplete, onError) {
  makeAsyncApiRequest(user, settings, function(response) {
    if (response.isValid()) {
      onComplete(response.getBody());
    } else {
      onError(response.getError())
    }
  });
}
updateUser(user, settings, function(body) { ... }, function(error) { ... });

In ES6, you may return a Promise which encapsulates the asynchronous request and either gets resolved or rejected, as shown in the following code:

function updateUser(user, settings) {
  return new Promise((resolve, reject) => {
    makeAsyncApiRequest(user, settings, function(response) {
      if (response.isValid()) {
        resolve(response.getBody());
      } else {
        reject(response.getError())
      }
    });
  });
}
updateUser(user, settings)
  .then(
    body => { ... },
    error => { ... }
  );

The real power of promises is that they can be passed around as objects, and promise handlers can be chained.