JS: Promise .prototype .then

By Xah Lee. Date: . Last updated: .

Promise .prototype .then

(new in ECMAScript 2015)

promise_object.then(onFulfilled,onRejected)

onFulfilled and onRejected are both functions of 1 parameter.

onFulfilled and onRejected are optional.

  • when the promise_object is fulfilled, onFulfilled is called with an arg xresult, where xresult is the arg passed to resolve function in the promise executor function body.
  • when the promise_object is rejected, onRejected is called with an arg xerror, where xerror is the arg passed to reject function in the promise executor function body.

The then-method returns a new promise object.

The new promise gets fulfilled if onFulfilled or onRejected runs successfully. gets rejected if onFulfilled or onRejected throws.