Promise in 50 lines of code

promise.js

var Promise = (function() {
    var Promise = function(callback) {
        this._done = [];
        this._fail = [];
        this.state = 'pending';
        this.value = undefined;
        callback && callback(this.resolve.bind(this), this.reject.bind(this));
    };
    Promise.prototype = {
        resolve: function(value) {
            this.state = 'resolved';
            this.value = value;
            this._done.forEach(function(each) {
                window.setTimeout(function() {
                    thenValue = each.callback(value);
                    if (thenValue && typeof thenValue.state === 'resolved') {
                        each.promise.resolve(thenValue.value)
                    }
                    else if (thenValue && typeof thenValue.state === 'rejected') {
                        each.promise.rejected(thenValue.value)
                    }
                    else if (thenValue && typeof thenValue.then === 'function') {
                        thenValue.then(each.promise.resolve.bind(each.promise),
                                       each.promise.reject.bind(each.promise));
                    } else {
                        each.promise.resolve(thenValue);
                    }
                });
            });
            return this;
        },
        reject: function(reason) {
            this.state = 'rejected';
            this.value = reason;
            this._fail.forEach(function(each) {
                window.setTimeout(function() {
                    thenValue = each.callback(reason);
                    if (thenValue && typeof thenValue.state === 'resolved') {
                        each.promise.resolve(thenValue.value)
                    }
                    else if (thenValue && typeof thenValue.state === 'rejected') {
                        each.promise.rejected(thenValue.value)
                    }
                    else if (thenValue && typeof thenValue.then === 'function') {
                        thenValue.then(each.promise.resolve.bind(each.promise),
                                       each.promise.reject.bind(each.promise));
                    } else {
                        each.promise.reject(thenValue);
                    }
                });
            });
            return this;
        },
        then: function(done, fail) {
            var newPromise = new Promise();
            switch (this.state) {
                case 'pending':
                    done && this._done.push({ callback: done, promise: newPromise });
                    fail && this._fail.push({ callback: fail, promise: newPromise });
                    break;
                case 'resolved':
                    newPromise.then(done, fail);
                    newPromise.resolve(this.value);
                    break;
                case 'rejected':
                    newPromise.then(done, fail);
                    newPromise.reject(this.value);
            }
            return newPromise;
        },
        
    };
    Promise.all = function(promises, values) {
        promises = promises || [];
        values = values || [];
        var newPromise =  new Promise();
        var first = promises.pop();
        if (first === undefined ) {
            newPromise.resolve(values);
        } else {
            first.then(function(value) {
                values.unshift(value);
                Promise.all(promises, values).then(newPromise.resolve.bind(newPromise));
            }, function(reason) {
                newPromise.reject(reason);
            });
        }
        return newPromise;
    }
    return Promise;
})();