Source code see here
Promise.prototype.then = function(onFulfilled, onRejected) {
if (this.constructor !== Promise) {
return safeThen(this, onFulfilled, onRejected);
}
var res = new Promise(noop);
handle(this, new Handler(onFulfilled, onRejected, res));
return res;
};
function safeThen(self, onFulfilled, onRejected) {
return new self.constructor(function (resolve, reject) {
//
var res = new Promise(noop);
res.then(resolve, reject);
handle(self, new Handler(onFulfilled, onRejected, res));
});
}