Learn ECMAScript - Second Edition

The then(onFulfilled, onRejected) method

The then() method of a Promise object lets us do a task after a Promise has been fulfilled or rejected. The task can also be another event-driven or callback-based asynchronous operation.

The then() method of a Promise object takes two arguments, that is, the onFulfilled and onRejected callbacks. The onFulfilled callback is executed if the Promise object was fulfilled, and the onRejected callback is executed if the Promise was rejected.

The onRejected callback is also executed if an exception is thrown in the scope of the executor. Therefore, it behaves like an exception handler, that is, it catches the exceptions. The onFulfilled callback takes a parameter, that is, the fulfilment value of the promise. ...