How to subclass a promise

angularpromise.js

// ES6

class AngularPromise extends Promise {
    constructor(executor) {
        super((resolve, reject) => {
            // before
            return executor(resolve, reject);
        });
        // after
    }

    then(onFulfilled, onRejected) {
        // before
        const returnValue = super.then(onFulfilled, onRejected);
        // after
        return returnValue;
    }
}

// ES5

function AngularPromise(executor) {
    var p = new Promise(function (resolve, reject) {
        // before
        return executor(resolve, reject);
    });
    // after

    p.__proto__ = AngularPromise.prototype;
    return p;
}

AngularPromise.__proto__ = Promise;
AngularPromise.prototype.__proto__ = Promise.prototype;

AngularPromise.prototype.then = function then(onFulfilled, onRejected) {
    // before
    var returnValue = Promise.prototype.then.call(this, onFulfilled, onRejected);
    // after
    return returnValue;
}

You don't have to define constructor argument If you don't need the value returned by .then method to be of your class instance.
Example:

class DeferredPromise extends Promise {
    static get [Symbol.species]() {
        return Promise;
    }
    constructor() {
        let internalResolve = () => { };
        let internalReject = () => { };
        super((resolve, reject) => {
            internalResolve = resolve;
            internalReject = reject;
        });
        this.resolve = internalResolve;
        this.reject = internalReject;
    }
}