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Promise.resolve() and Promise.reject()

The static function Promise.resolve(value) or Promise.reject(value) returns a promise that is resolved or rejected respectively, with the given value.


Promise.reject(100).then(
  console.log,
  n => { console.log(n * 2) }
);

200

Passing a Promise object to Promise.resolve() is identical to calling the promise directly.


var p = new Promise(resolve => {
  resolve(100);
});
console.log(Promise.resolve(p) === p);
Promise.resolve(p).then(
  console.log,
  n => { console.log(n * 2) }
);

true 100

Passing to Promise.resolve() a thenable (an object with a 'method' taking two callbacks) is identical to calling a promise with the two callbacks in the thenable.


var thenable = {
  then: (onFulfill, onReject) => { onFulfill('fulfilled'); }
};
Promise.resolve(thenable).then(console.log);

var p = new Promise(resolve => { resolve('fulfilled'); });  // just identical to the above
p.then(console.log);

fulfilled fulfilled