My second attempt at Javascript Promises (aka Futures), in native Javascript

Promise.js

/**
 * Implementation of Javascript Promises.
 * A simplified version of the U4.Promise framework I wrote for Unit4 in 2013.
 * Building it up as I need it.
 */
var Promise = (function(){
    /**
     * @private
     * Copy properties from one object to another.
     * @param target
     * @param source
     */
    function applyProperties(target, source) {
        for(var key in source) {
            if(source.hasOwnProperty(key)) {
                target[key] = source[key];
            }
        }
    }

    /**
     * Call handler for every own property in obj. Works on arrays and objects
     * @param obj
     * @param handler
     * @param thisArg
     */
    function each (obj, handler, thisArg) {
        var key;
        for (key in obj){
            if(obj.hasOwnProperty(key)){
                handler.call(thisArg || this, obj[key]);
            }
        }
    }

    /**
     * Call handler for every own property in obj, until handler returns truthy.
     * @param obj
     * @param handler
     * @param thisArg
     */
    function some (obj, handler, thisArg) {
        var key;
        for (key in obj){
            if(obj.hasOwnProperty(key)){
                if(handler.call(thisArg || this, obj[key])){
                    break;
                }
            }
        }
    }

    /**
     * Create a copy of obj, applying a transform to each own property.
     * @param obj
     * @param transform
     * @param thisArg
     * @returns {Array}
     */
    function map (obj, transform, thisArg) {
        var result = Array.isArray(obj) ? [] : {},
            key;

        for (key in obj){
            if(obj.hasOwnProperty(key)){
                result[key] = transform.call(thisArg || this, obj[key]);
            }
        }

        return result;
    }

    /**
     * @param x
     * @returns {*|boolean|Boolean} true if x is a Promise.
     */
    function isPromise(x) {
        return x && x.isPromise;
    }

    /**
     * Do nothing.
     */
    function noOperation() {}

    /**
     * Base Promise class.
     * @constructor
     */
    function Promise() {
        this._promiseConstructor();
        return this;
    }

    Promise.prototype = {
        /** @property {Boolean} isPromise marker to say it's a promise. */
        isPromise: true,
        /** @property {Boolean} resolved true if the promise is resolved. */
        resolved: false,
        /** @property {Boolean} rejected true if the promise is rejected. */
        rejected: false,
        /** @property {*} result result if we have one. */
        result: undefined,
        /** @property {*} error error if we have one. */
        error: undefined,

        _promiseConstructor : function () {
            /** @property {Promise[]} _next ongoing promises in success case. */
            this._next = [];
            /** @property {Promise[]} _otherwise ongoing promises in fail case. */
            this._otherwise = [];
        },

        /**
         * Register something to do when the promise resolves.
         * @param {Function} handler handler function to call.
         * @param {*} handler.result the upstream result
         * @param {Promise|*} handler.returns handler returns its result or a Promise of its result.
         * @param {Object} [scope] scope to call handler in.
         * @returns {Promise} promise of the handler's completion.
         */
        then: function (handler, scope) {
            return this.thenResolve(new OngoingPromise(handler, scope));
        },

        /**
         * Register another Promise as something to do when this promise resolves.
         * The Promise will be rejected if this promise rejects.
         * @param downstreamPromise
         * @returns {*}
         */
        thenResolve: function (downstreamPromise) {
            var me = this;

            if(me.resolved) {
                downstreamPromise.onUpstreamResolved(me.result);
            } else if (me.rejected) {
                downstreamPromise.onUpstreamRejected(me.error);
            } else {
                me._next.push(downstreamPromise);
            }
            return downstreamPromise;
        },

        /**
         * Register something to do when the promise Rejects.
         * @param handler
         * @param [scope]
         * @returns {*}
         */
        otherwise: function (handler, scope) {
            return this.otherwiseResolve(new OngoingPromise(handler, scope));
        },

        /**
         * Register another Promise as something to do when this promise rejects.
         * The Promise will only be resolved if this promise or an upstream rejects.
         * @param {Promise} downstreamPromise
         */
        otherwiseResolve: function (downstreamPromise) {
            var me = this;

            if (me.rejected) {
                downstreamPromise.onUpstreamResolved(me.error);
            } else if (!me.resolved) {
                me._otherwise.push(downstreamPromise);
            }
            return downstreamPromise;
        },

        /**
         * Register something to do regardless of the result of the promise.
         * @param {Function} handler function to call
         * @param {Object} handler.arg composite containing the result
         * @param {Object} handler.arg.result success result.
         * @param {Object} handler.arg.error error result.
         * @param {Boolean} handler.arg.isSuccess true if success
         * @param {Object} [scope] scope for the handler
         * @returns {Promise} promise of the handler completing.
         */
        thenAlways: function (handler, scope) {
            return this.alwaysResolve(new OngoingPromise(handler, scope));
        },

        /**
         * Register a promise to call regardless of the outcome of this promise.
         * The result will be a composite as described in {@link #thenAlways}.
         * @param {Promise} downstreamPromise
         */
        alwaysResolve: function (downstreamPromise) {
            this.then(function(result){
                downstreamPromise.onUpstreamResolved({
                    isSuccess: true,
                    result: result
                });
            });

            this.otherwise(function(error){
                downstreamPromise.onUpstreamResolved({
                    isSuccess: false,
                    error: error
                });
            });

            return downstreamPromise;
        },

        /**
         * If an error transforms through this point then it can be transformed.
         * Allows something like "The warp drive is broken." to be transformed to
         * "Problem jumping into hyperspace. The warp drive is broken."
         *
         * @param {Function} transform method to transform the error
         * @param {*} transform.error the error being passed.
         * @param {!Promise} transform.return the transformed error. This must be synchronous.
         * @param {Object} [scope] scope for the transform function.
         */
        transformError: function (transform, scope) {
            return this.thenResolve(new TransformErrorPromise(transform, scope));
        },

        /**
         * @protected
         * Handle an upstream promise resolving.
         * The base class immediately rejects itself.
         * @param {*} error the error from the upstream promise.
         */
        onUpstreamResolved: function (result) {
            this.resolve(result);
        },

        /**
         * @protected
         * Handle an upstream promise rejecting.
         * The base class immediately resolves itself.
         * @param {*} result the result from the upstream promise.
         */
        onUpstreamRejected: function (error) {
            this.reject(error);
        },

        /**
         * Resolve the Promise. Can only be called once.
         * @param {Promise|*} [result] the result or the promise of the result.
         */
        resolve: function (result) {
            if (result && result.isPromise) {
                result.thenResolve(this);
            } else {
                this.resolved = true;
                this.result = result;
                this._next.forEach(function (promise) {
                    promise.onUpstreamResolved(result);
                });
                this.inhibitFutureInput();
            }
        },

        inhibitFutureInput: function() {
            delete this._next;
            delete this._otherwise;
            this.resolve = noOperation;
            this.reject = noOperation;
        },

        /**
         * Reject the Promise. Can only be called once.
         * @param {*|!Promise} [error] error result. Cannot be a promise.
         */
        reject: function (error) {
            this.rejected = true;
            this.error = error;
            this._next.forEach(function (promise) {
                promise.onUpstreamRejected(error);
            });
            this._otherwise.forEach(function (promise) {
                promise.onUpstreamResolved(error);
            });
            this.inhibitFutureInput();
        },

        /**
         * @return {Boolean} true if resolved or rejected
         */
        isComplete: function() {
            return this.resolved || this.rejected;
        }
    };

    /**
     * @static
     * Convenience function to create a rejected promise. When returned in a Promise handler, causes a
     * reject.
     * @param error the error to reject with.
     */
    Promise.reject = function(error) {
        var promise = new Promise();
        promise.reject(error);
        return promise;
    };

    /**
     * @static
     * Convert an unknown into a Promise.
     * @param {Promise|*} result if a Promise then wait on that promise, otherwise return a promise resolved with the result.
     * @returns {Promise}
     */
    Promise.resolve = function(result) {
        var promise;
        if(isPromise(result)){
            return result;
        }
        promise = new Promise();
        promise.resolve(result);
        return promise;
    };

    /**
     * @private a promise with a handler, downstream of an original promise.
     * @param handler
     * @param scope
     * @constructor
     */
    function OngoingPromise(handler, scope){
        this._promiseConstructor();
        this.handler = handler;
        this.scope = scope;
    }

    OngoingPromise.prototype = new Promise();
    applyProperties(OngoingPromise.prototype, {
        onUpstreamResolved: function(result) {
            var nextResult;
            try {
                nextResult = this.handler.call(this.scope || this, result);
                // If we re-enter onUpstreamResolved it is because nextResult
                // was a Promise, so we want to resolve immediately.
                this.onUpstreamResolved = this.resolve;
                this.resolve(nextResult);
            } catch (e) {
                this.reject(e);
            }
        }
    });

    /**
     * @private
     * A promise that transforms errors that pass through it.
     * @param {Function} transform
     * @param {Object} [scope]
     * @constructor
     */
    function TransformErrorPromise(transform, scope){
        this._promiseConstructor();
        this.transform = transform;
        this.scope = scope;
    }

    TransformErrorPromise.prototype = new Promise();
    applyProperties(TransformErrorPromise.prototype, {
        onUpstreamRejected: function (error) {
            var newError;
            try {
                newError = this.transform.call(this.scope || this, error);
            } catch (e) {
                newError = 'Error transforming error. Original error was: ' + error;
            }
            this.reject(newError);
        }
    });

    /**
     * @static
     * Create a Promise which resolves when all requirements have resolved. **Fast Reject Version**
     *
     * The Result will be a Map or Array matching Requirements with the Promise results in each place.
     * If Rejected then the Error that from the first found rejection. The Promise will reject as soon as
     * an upstream Promise rejects.
     *
     * @param {Object|Array} requirements promises or results to wait for.
     * then we will wait and collect all results.
     *
     * If fastReject is true then the rejection passed down will be the first one encountered with no information about
     * where it came from. If it is false then the rejection will be an Object/Array of promises based on the original
     * requirement to allow interrogation.
     */
    Promise.when = function(requirements) {
        return new JoinPromise(requirements, true);
    };

    /**
     * @static
     * Create a Promise which resolves when all requirements have  **Slow Reject Version**
     *
     * The Result will be a Map or Array matching Requirements with the Promise results in each place.
     * If Rejected then the Error will be the same Map or Array of each Promise. Promises will be created
     * to wrap non-Promise inputs.
     *
     * @param {Object|Array} requirements promises or results to wait for.
     * then we will wait and collect all results.
     *
     * If fastReject is true then the rejection passed down will be the first one encountered with no information about
     * where it came from. If it is false then the rejection will be an Object/Array of promises based on the original
     * requirement to allow interrogation.
     */
    Promise.whenAllWithSlowReject = function(requirements) {
        return new JoinPromise(requirements, false);
    };

    /**
     * @private
     * @param requirements
     * @param fastReject
     * @constructor
     */
    function JoinPromise(requirements, fastReject) {
        this.requirements = requirements;
        this.fastReject = !!fastReject;
        this.onUpstreamStateChange();

        each(requirements, function(requirement){
            if (isPromise(requirement)) {
                // This comes into our upstreamResolve method which is overridden
                // to calculate the real state.
                requirement.alwaysResolve(this);
            }
        }, this)
    }

    JoinPromise.prototype = new Promise();
    applyProperties(JoinPromise.prototype, {
        onUpstreamStateChange: function () {
            var allOk = true,
                allComplete = true,
                someError = false,
                requirements = this.requirements;

            if(!this.isComplete()){
                each(requirements, function(requirement){
                    if(isPromise(requirement)) {
                        allOk &= requirement.resolved;
                        allComplete &= (requirement.rejected || requirement.resolved);
                        someError |= requirement.rejected;
                    }
                });

                if(allOk){
                    this.doResolve();
                } else if(allComplete || (this.fastReject && someError)){
                    this.doReject();
                }
            }
        },

        doResolve: function(){
            this.resolve(map(this.requirements, function(requirement){
                return isPromise(requirement) ? requirement.result : requirement;
            }));
        },

        doReject: function(){
            if(this.fastReject){
                // Reject with the first error found.
                var error = undefined;
                some(this.requirements, function(requirement){
                    if(requirement && isPromise(requirement) && requirement.rejected){
                        error = requirement.error;
                        return true;
                    }
                });
                this.reject(error);
            } else {
                // Reject with full information.
                this.reject(map(this.requirements, function(requirement){
                    return Promise.resolve(requirement);
                }));
            }
        },

        onUpstreamResolved: function(){
            this.onUpstreamStateChange();
        },

        onUpstreamRejected: function(){
            this.onUpstreamStateChange();
        }
    });

    return Promise;
})();

Promise.spec.js

describe('Promise', function() {

    /** Promise handler to add one to the input. */
    function addOne(x){return x+1;}
    /** Promise handler to multiply the input by two. */
    function timesTwo(x){return x*2; }
    /** Promise handler that throws 'An Exception' */
    function throwException(x){throw 'An Exception';}
    /** Promise handler that returns a Rejected promise with message 'Reject'. */
    function returnReject(x){return Promise.reject('Reject');}

    /** Create a promise handler to store its input on its scope with the given key, and pass that input on. */
    function storeResult(key){
        return function(x){this[key] = x; return x;}
    }

    /** Create a prmise handler to increment a counter in its scope, then pass its input on. */
    function increment(key){
        return function(){this[key]++; return x;}
    }

    function assertPromiseReady(promise){
        expect(promise.resolved).toBe(false, 'assertPromiseReady: expected resolved to be false');
        expect(promise.rejected).toBe(false, 'assertPromiseReady: rejected resolved to be false');
        expect(promise.isComplete()).toBe(false,  'assertPromiseReady: expected isComplete() to be false');
        expect(promise.result).toBeUndefined('assertPromiseReady: expected result to be undefined');
        expect(promise.error).toBeUndefined('assertPromiseReady: expected error to be undefined');
    }

    function assertPromiseResolved(promise, expectedResult) {
        expect(promise.resolved).toBe(true, 'assertPromiseResolved: expected resolved to be true');
        expect(promise.rejected).toBe(false, 'assertPromiseResolved: rejected resolved to be false');
        expect(promise.isComplete()).toBe(true,  'assertPromiseResolved: expected isComplete() to be true');
        expect(promise.result).toEqual(expectedResult, 'assertPromiseResolved: expected result');
        expect(promise.error).toBeUndefined('assertPromiseResolved: expected error to be undefined');
    }

    function assertPromiseRejected(promise, expectedError) {
        expect(promise.resolved).toBe(false, 'assertPromiseRejected: expected resolved to be false');
        expect(promise.rejected).toBe(true, 'assertPromiseRejected: expected rejected to be true');
        expect(promise.isComplete()).toBe(true,  'assertPromiseRejected: expected isComplete() to be true');
        expect(promise.result).toBeUndefined('assertPromiseRejected: expected result to be undefined');
        expect(promise.error).toEqual(expectedError, 'assertPromiseRejected: expected error');
    }

    describe('new Promise()', function() {
        it('is initially unresolved and unrejected', function () {
            var p = new Promise();
            assertPromiseReady(p);
        });
    });

    describe('resolve()', function(){
        it('becomes resolved and complete once resolved', function(){
            var p = new Promise();
            p.resolve('Fred');
            assertPromiseResolved(p, 'Fred');
        });

        it('can accept no argument as an answer', function(){
            var p = new Promise();
            p.resolve();
            assertPromiseResolved(p, undefined);
        });

        it('can accept a complex argument as an answer', function(){
            var p = new Promise(),
                expectedResult = {foo:'bar'};

            p.resolve(expectedResult);
            assertPromiseResolved(p, expectedResult);
            expect(p.result).toBe(expectedResult); // No clones
        });

        it('can be resolved with a Promise, so allowing casting to Promise', function(){
            var promise1 = new Promise(),
                promise2 = new Promise();

            promise1.then(timesTwo).then(storeResult('result'), this);
            promise1.resolve(promise2);
            assertPromiseReady(promise1);

            promise2.resolve(5);
            assertPromiseResolved(promise1, 5); // Initial value resolved with
            expect(this.result).toBe(10); // End result of the chain.
        });

        it('can be resolved with a Promise, fires immediately if the Promise is already resolved', function(){
            var promise1 = new Promise(),
                promise2 = new Promise();

            promise2.resolve(5);

            promise1.then(timesTwo).then(storeResult('result'), this);
            promise1.resolve(promise2);

            assertPromiseResolved(promise1, 5); // Initial value resolved with
            expect(this.result).toBe(10); // End result of the chain.
        });

        it('if already resolved resists further attempts to resolve it', function(){
            var p = new Promise(),
                result = {
                    count: 0
                };

            p.then(storeResult('result'), result).then(increment('count'), result);
            p.resolve('x');
            expect(result).toEqual({count: 1, result: 'x'});
            p.resolve('y');
            expect(result).toEqual({count: 1, result: 'x'});
            assertPromiseResolved(p, 'x');
        });

        it('if already rejected resists further attempts to resolve it', function(){
            var p = new Promise(),
                result = {
                    count: 0
                };

            p.then(storeResult('result'), result).then(increment('count'), result);
            p.reject('x');
            p.resolve('y');
            expect(result).toEqual({count: 0});
            assertPromiseRejected(p, 'x');
        });
    });

    describe('reject()', function(){
        it('becomes rejected and complete once rejected', function(){
            var p = new Promise();
            p.reject('An Error');
            assertPromiseRejected(p, 'An Error');
        });

        it('can accept no argument as an error', function(){
            var p = new Promise();
            p.reject();
            assertPromiseRejected(p, undefined);
        });

        it('can accept a complex argument as an answer', function(){
            var p = new Promise(),
                expectedResult = {error: true, msg:'bar'};

            p.reject(expectedResult);
            assertPromiseRejected(p, expectedResult);
            expect(p.error).toBe(expectedResult); // No clones
        });

        it('if already resolved resists further attempts to reject it', function(){
            var p = new Promise(),
                result = {
                    count: 0
                };

            p.otherwise(storeResult('error'), result).then(increment('count'), result);
            p.reject('x');
            expect(result).toEqual({count: 1, error: 'x'});
            p.reject('y');
            expect(result).toEqual({count: 1, error: 'x'});
            assertPromiseRejected(p, 'x');
        });

        it('if already rejected resists further attempts to resolve it', function(){
            var p = new Promise(),
                result = {
                    count: 0
                };

            p.then(storeResult('result'), result).then(increment('count'), result)
                .otherwise(storeResult('error'), result).then(increment('count'), result);

            p.reject('x');
            expect(result).toEqual({count: 1, error: 'x'});
            p.resolve('y');
            expect(result).toEqual({count: 1, error: 'x'});
            assertPromiseRejected(p, 'x');
        });
    });

    describe('then()', function() {
        it('can call multiple handlers once resolved', function () {
            var p = new Promise(),
                result1 = '--UNSET--',
                result2 = '--UNSET--';

            p.then(function (x) {
                result1 = x;
            });
            p.then(function (x) {
                result2 = x;
            });

            expect(result1).toBe('--UNSET--');
            p.resolve(12);
            expect(result1).toBe(12);
            expect(result2).toBe(12);
        });

        it('accepts a scope parameter for handlers', function () {
            var p = new Promise(),
                capturedScope = '--UNSET--',
                requiredScope = {foo: 'Bar'};

            p.then(function () {
                capturedScope = this
            }, requiredScope);
            p.resolve(12);
            expect(capturedScope).toBe(requiredScope);
        });

        it('calls an added handler immediately if already resolved', function () {
            var p = new Promise(),
                result = '--UNSET--';

            p.resolve(34);
            p.then(function (x) {
                result = x;
            });
            expect(result).toBe(34);
        });

        it('does not call its handlers once rejected', function () {
            var p = new Promise(),
                result = '--UNSET--';

            p.then(function (x) {
                result = x;
            });

            p.reject('An Error');
            expect(result).toBe('--UNSET--');
        });
    });

    describe('otherwise()', function() {
        it('can call multiple error handlers once rejected', function () {
            var p = new Promise(),
                result1 = '--UNSET--',
                result2 = '--UNSET--';

            p.otherwise(function (x) {
                result1 = x;
            });
            p.otherwise(function (x) {
                result2 = x;
            });

            expect(result1).toBe('--UNSET--');
            p.reject('An Error');
            expect(result1).toBe('An Error');
            expect(result2).toBe('An Error');
        });

        it('accepts a scope parameter for handlers', function () {
            var p = new Promise(),
                capturedScope = '--UNSET--',
                requiredScope = {foo: 'Bar'};

            p.otherwise(function () {
                capturedScope = this
            }, requiredScope);

            p.reject('An Error');
            expect(capturedScope).toBe(requiredScope);
        });

        it('calls an added error handler immediately once rejected', function () {
            var p = new Promise(),
                result = '--UNSET--';

            p.reject('An Error');
            p.otherwise(function(x){
                result = x;
            });

            expect(result).toBe('An Error');
        });

        it('does not call its error handlers once resolved', function () {
            var p = new Promise(),
                result = '--UNSET--';

            p.otherwise(function (x) {
                result = x;
            });

            p.resolve('The Result');
            expect(result).toBe('--UNSET--');
        });
    });

    describe('thenAlways()', function() {
        it('can call a handler for resolve and reject, resolve case', function(){
            var p = new Promise(),
                result = '--UNSET--';

            p.thenAlways(function(x){
                result = x;
            });

            p.resolve('Result');
            expect(result).toEqual({
                result: 'Result',
                isSuccess: true
            });
        });

        it('can call a handler for resolve and reject, resolve case - previously resolved', function(){
            var p = new Promise(),
                result = '--UNSET--';

            p.resolve('Result');

            p.thenAlways(function(x){
                result = x;
            });

            expect(result).toEqual({
                result: 'Result',
                isSuccess: true
            });
        });

        it('can call a handler for resolve and reject, reject case', function(){
            var p = new Promise(),
                result = '--UNSET--';

            p.thenAlways(function(x){
                result = x;
            });

            p.reject('An Error');

            expect(result).toEqual({
                error: 'An Error',
                isSuccess: false
            });
        });

        it('can call a handler for resolve and reject, reject case - previously rejected', function(){
            var p = new Promise(),
                result = '--UNSET--';

            p.reject('An Error');

            p.thenAlways(function(x){
                result = x;
            });

            expect(result).toEqual({
                error: 'An Error',
                isSuccess: false
            });
        });

        // These were not expected to work seeing the code. They were found to work
        // because of work done to prevent re-entry of Promises. As long as all tests
        // in this section pass then we can be confident. TDD means we don't need to
        // solve it as it already works.
        it('does not call the handler twice on reject', function(){
            var p = new Promise(),
                result = {count: 0};

            p.thenAlways(increment('count'), result);
            p.reject('Foo')
            expect(result.count).toBe(1);
        });

        it('does not call the handler twice on prior reject', function(){
            var p = new Promise(),
                result = {count: 0};

            p.reject('Foo')
            p.thenAlways(increment('count'), result);
            expect(result.count).toBe(1);
        });

        it('does not call the handler twice on prior resolve', function(){
            var p = new Promise(),
                result = {count: 0};

            p.resolve('Foo')
            p.thenAlways(increment('count'), result);
            expect(result.count).toBe(1);
        });
    });

    describe('Promise Chaining', function(){
        it('Promises can be chained to produce a new Promise of the result of its handler', function(){
            var firstPromise = new Promise(),
                lastPromise = firstPromise.then(addOne).then(timesTwo).then(storeResult('Result'), this);

            firstPromise.resolve(10);

            expect(this.Result).toBe(22);
            assertPromiseResolved(lastPromise, 22);
        });

        it('Promises can be chained to produce a new Promise of the result of its handler - Previously Resolve Case', function(){
            var firstPromise = new Promise(),
                lastPromise;

            firstPromise.resolve(10);
            lastPromise = firstPromise.then(addOne).then(timesTwo).then(storeResult('Result'), this);

            expect(this.Result).toBe(22);
            assertPromiseResolved(lastPromise, 22);
        });

        it('propagates rejects down the chain', function(){
            var firstPromise = new Promise(),
                lastPromise = firstPromise.then(addOne).then(timesTwo),
                error = { error: true, message: 'An Error' };

            lastPromise.otherwise(storeResult('capturedError'), this);
            firstPromise.reject(error);

            expect(this.capturedError).toBe(error); // Not a clone.
            assertPromiseRejected(lastPromise, error);
        });

        it('A Promise can be rejected by throwing an exception in the handler', function(){
            var firstPromise = new Promise(),
                lastPromise = firstPromise.then(throwException).then(timesTwo);

            lastPromise.otherwise(storeResult('capturedError'), this);
            firstPromise.resolve(10);

            expect(this.capturedError).toBe('An Exception');
            assertPromiseRejected(lastPromise, 'An Exception');
        });

        it('A Promise can be rejected by returning Promise.reject() in the handler', function(){
            var firstPromise = new Promise(),
                lastPromise = firstPromise.then(returnReject).then(timesTwo);

            lastPromise.otherwise(storeResult('capturedError'), this);
            firstPromise.resolve(10);

            expect(this.capturedError).toBe('Reject');
            assertPromiseRejected(lastPromise, 'Reject');
        });

        it('will wait for a Promise returned by a handler to be resolved, then resolve', function(){
            var initialPromise = new Promise(),
                innerPromise = new Promise(),
                finalPromise;

            finalPromise = initialPromise.then(function(){return innerPromise;}).then(storeResult('result'), this);

            assertPromiseReady(finalPromise);
            initialPromise.resolve('Outer Result');
            assertPromiseReady(finalPromise);
            innerPromise.resolve('Inner Result');
            assertPromiseResolved(finalPromise, 'Inner Result');
            expect(this.result).toBe('Inner Result');
        });

        it('will all a hander to return a pre-resolved promise, then resolve', function(){
            var initialPromise = new Promise(),
                innerPromise = new Promise(),
                finalPromise;

            innerPromise.resolve('Inner Result');
            finalPromise = initialPromise.then(function(){return innerPromise;}).then(storeResult('result'), this);

            assertPromiseReady(finalPromise);
            initialPromise.resolve('Outer Result');
            assertPromiseResolved(finalPromise, 'Inner Result');
            expect(this.result).toBe('Inner Result');
        });

        it('will wait for a Promise returned by a handler to be rejected, then reject', function(){
            var initialPromise = new Promise(),
                innerPromise = new Promise(),
                finalPromise;

            finalPromise = initialPromise.then(function(){return innerPromise;});
            finalPromise.otherwise(storeResult('capturedError'), this);

            initialPromise.resolve();
            innerPromise.reject('Inner Error');
            assertPromiseRejected(finalPromise, 'Inner Error');
            expect(this.capturedError).toBe('Inner Error');
        });
    });

    describe('thenResolve()', function() {
        it('can be asked to resolve another promise when resolved. Returns that Promise', function () {
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results);
            promise1.thenResolve(promise2).then(timesTwo).then(storeResult('endResult'), results);

            assertPromiseReady(promise1);
            assertPromiseReady(promise2);

            promise1.resolve(2);
            assertPromiseResolved(promise1, 2); // Initial value resolved with
            assertPromiseResolved(promise2, 2); // Initial value resolved with
            expect(results.p2).toBe(2); // Promise2 is resolved with the result of Promise1
            expect(results.endResult).toBe(4);
        });

        it('resolves immediately if the promise is already resolved', function () {
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results);

            promise1.resolve(2);
            promise1.thenResolve(promise2).then(timesTwo).then(storeResult('endResult'), results);

            assertPromiseResolved(promise1, 2); // Initial value resolved with
            assertPromiseResolved(promise2, 2); // Initial value resolved with
            expect(results.p2).toBe(2); // Promise2 is resolved with the result of Promise1
            expect(results.endResult).toBe(4);
        });

        it('rejects the other promise if this promise is rejected', function(){
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results).otherwise(storeResult('p2Error'), results);
            promise1.thenResolve(promise2);

            promise1.reject('An Error');
            assertPromiseRejected(promise1, 'An Error');
            assertPromiseRejected(promise2, 'An Error');
            expect(results.p2).toBeUndefined();
            expect(results.p2Error).toBe('An Error');
        });

        it('rejects the other promise immediately if this promise is already rejected', function(){
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results).otherwise(storeResult('p2Error'), results);

            promise1.reject('An Error');
            promise1.thenResolve(promise2);

            assertPromiseRejected(promise1, 'An Error');
            assertPromiseRejected(promise2, 'An Error');
            expect(results.p2).toBeUndefined();
            expect(results.p2Error).toBe('An Error');
        });

    });

    describe('otherwiseResolve()', function(){
        it('can be asked to resolve another promise when rejected. Returns that Promise', function(){
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results);
            promise1.otherwiseResolve(promise2).then(timesTwo).then(storeResult('endResult'), results);

            assertPromiseReady(promise1);
            assertPromiseReady(promise2);

            promise1.reject(2);
            assertPromiseRejected(promise1, 2); // Initial value resolved with
            assertPromiseResolved(promise2, 2); // Initial value resolved with
            expect(results.p2).toBe(2); // Promise2 is resolved with the result of Promise1
            expect(results.endResult).toBe(4);
        });

        it('rejects immediately if the promise is already rejected', function(){
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results);

            promise1.reject(2);
            promise1.otherwiseResolve(promise2).then(timesTwo).then(storeResult('endResult'), results);

            assertPromiseRejected(promise1, 2); // Initial value resolved with
            assertPromiseResolved(promise2, 2); // Initial value resolved with
            expect(results.p2).toBe(2); // Promise2 is resolved with the result of Promise1
            expect(results.endResult).toBe(4);
        });

        /**
         * Rejection is assumed for error handling not flow control, so we don't start off rejecting
         * an error chain in the inital promise succeeded.
         */
        it('does not reject the "otherwise Promise" when resolved', function(){
            var promise1 = new Promise(),
                promise2 = new Promise(),
                results = {};

            promise2.then(storeResult('p2'), results);
            promise1.otherwiseResolve(promise2).then(timesTwo).then(storeResult('endResult'), results);

            assertPromiseReady(promise1);
            assertPromiseReady(promise2);

            promise1.resolve(2);
            assertPromiseResolved(promise1, 2); // Initial value resolved with
            assertPromiseReady(promise2); // Never completes.
            expect(results.endResult).toBeUndefined();
        });
    });

    describe('transformError()', function(){
        it('can transform an error passing through a chain of Promises', function(){
            var externalSystemCall = new Promise();

            function someServiceCall(){
                return externalSystemCall
                    .then(addOne)
                    .transformError(function(e){
                        return 'Problem adding one to an external result: ' + e;
                    });
            }

            someServiceCall().otherwise(storeResult('capturedFinalError'), this);
            externalSystemCall.reject('External Service Error');
            expect(this.capturedFinalError).toBe('Problem adding one to an external result: External Service Error');
        });
    });

    describe('Promise.resolve() - can be used to turn an unknown into a Promise', function(){
        it('accepts a value and returns a resolved promise of that value', function(){
            var p = Promise.resolve(2);
            assertPromiseResolved(p, 2);
        });

        it('accepts no argument and returns a resolved promise of undefined value', function(){
            var p = Promise.resolve();
            assertPromiseResolved(p, undefined);
        });

        it('accepts a Promise and returns that same Promise', function(){
            var p1 = new Promise(),
                p2 = Promise.resolve(p1);
            expect(p2).toBe(p1);
        });
    });

    describe('Promise.when() - Joining promises', function(){
        it('can wait on multiple Promises, success case, Object requirements', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when({
                    p1: p1,
                    p2: p2,
                    p3: p3
                });

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseReady(joined);
            p2.resolve(2);
            assertPromiseResolved(joined, {
                p1: 1,
                p2: 2,
                p3: 3
            });
        });

        it('can wait on a mix of Promises and literals, success case, Object requirements', function(){
            var p1 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when({
                    p1: p1,
                    literal: 'Hello',
                    p3: p3
                });

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseResolved(joined, {
                p1: 1,
                literal: 'Hello',
                p3: 3
            });
        });

        it('can wait on a mix of Promises and literals when a literal is undefined', function(){
            var p1 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when({
                    p1: p1,
                    literal: undefined,
                    p3: p3
                });

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseResolved(joined, {
                p1: 1,
                literal: undefined,
                p3: 3
            })

        });

        it('can wait on multiple Promises, success case, Array requirements', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when([p1, p2, p3]);

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseReady(joined);
            p2.resolve(2);
            assertPromiseResolved(joined, [1, 2, 3]);
        });

        it('can wait on multiple Promises, success case, Array requirements with literals', function(){
            var p1 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when([p1, 2, p3]);

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseResolved(joined, [1, 2, 3]);
        });

        it('can wait on multiple Promises, success case, Object requirements, all previously resolved', function(){
            var p = Promise.when({
                p1: Promise.resolve(1),
                p2: Promise.resolve(2),
                pu: Promise.resolve(),
                l1: 'Literal'
            });

            assertPromiseResolved(p, {
                p1: 1,
                p2: 2,
                pu: undefined,
                l1: 'Literal'
            });
        });

        it('can wait on multiple Promises, error case, slow reject, Object requirements - rejects with Object containing Promises', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.whenAllWithSlowReject({
                    p1: p1,
                    p2: p2,
                    p3: p3,
                    l1: 'Literal'
                });

            assertPromiseReady(joined);
            p1.reject('E1');
            assertPromiseReady(joined);
            p2.resolve(2);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseRejected(joined, {
                p1: p1,
                p2: p2,
                p3: p3,
                l1: Promise.resolve('Literal')
            });
        });

        it('can wait on multiple Promises, error case, slow reject, Array Requirements - rejects with Promise array', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.whenAllWithSlowReject([p1, p2, p3, 'Literal']);

            assertPromiseReady(joined);
            p1.reject('E1');
            assertPromiseReady(joined);
            p2.resolve(2);
            assertPromiseReady(joined);
            p3.resolve(3);
            assertPromiseRejected(joined, [p1, p2, p3, Promise.resolve('Literal')]);
        });

        it('can wait on multiple Promises, error case, fast reject, Object requirements - rejects with single error', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when({
                    p1: p1,
                    p2: p2,
                    p3: p3,
                    l1: 'Literal'
                });

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p2.reject('E2');
            assertPromiseRejected(joined, 'E2');
        });

        it('can wait on multiple Promises, error case, fast reject, Array requirements - rejects with single error', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                joined = Promise.when([p1, p2, p3, 'Literal']);

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p2.reject('E2');
            assertPromiseRejected(joined, 'E2');
        });

        it('can wait on multiple Promises - once fast reject has fired future results are ignored', function(){
            var p1 = new Promise(),
                p2 = new Promise(),
                p3 = new Promise(),
                p4 = new Promise(),
                joined = Promise.when([p1, p2, p3, 'Literal', p4]),
                captureCount = 0;

            function capture(){
                captureCount++;
            }

            joined.thenAlways(capture);

            assertPromiseReady(joined);
            p1.resolve(1);
            assertPromiseReady(joined);
            p2.reject('E2');
            assertPromiseRejected(joined, 'E2');
            p3.reject('E3');
            assertPromiseRejected(joined, 'E2');
            p4.resolve(4);
            assertPromiseRejected(joined, 'E2');

            expect(captureCount).toBe(1);
        });

        it('can wait on multiple Promises, error case, Object requirements, all previously rejected - rejects immediately', function(){
            var p1 = Promise.reject('E1'),
                p2 = Promise.reject('E2');

            Promise.whenAllWithSlowReject({
                    p1: p1,
                    p2: p2
            }).otherwise(storeResult('result'), this);

            expect(this.result).toEqual({
                p1: p1,
                p2: p2
            });
        });

        it('can wait on multiple Promises, error case, Object requirements, fast reject, some previously rejected - rejects immediately', function(){
            var result = {};

            Promise.when({
                p1: Promise.reject('E1'),
                p2: new Promise(),
                l1: 'Literal'
            }).otherwise(storeResult('error'), result);

            expect(result.error).toBe('E1');
        });
    });
});

I've spent some time at work porting this to Ext-JS and in doing some found some ways it can be better. It all passes the unit tests (assuming I've uploaded latest which I can do) but there are things that can be Better. Given time I can make the tweaks - notably:

• The subclass prototypes get the Promise initialiser, so carry the lists. Either lazy instantiate the lists or delete them from the prototype. Does not cause a problem.
• A condition rather than overwriting method pointers makes sense to inhibit future firing. It was an interesting idea, but the condition more understandable.
• The then method can better follow Promises-A and take a fail handler as second argument allowing then(handler, [failHandler], [scope])