Promise

实现一个符合 Promise/A+ 规范的 MyPromise，并实现 resolve、reject、all、race、defer、deferred等静态方法。

MyPromise

1. 作用：创建 MyPromise实例。Promise
2. MyPromise接收一个回掉函数 executor
3. MyPromise状态
   • pending
     • 可以转换成 fulfilled 或 rejected
   • fulfilled
     • 不可改变成其他状态
   • rejected
     • 不可改变成其他状态
4. onFulfilledCallbacks 和 onRejectedCallbacks
   • 两个数组，数组每一项是一个函数。分别接收then里面的第一个参数和第二个参数。
   • 状态是 pending 的回掉函数。
5. resolve
   • promise的状态是fulfilled异常是的处理函数
   • 接收 value 参数
     • 如果是promise，执行then。
     • 如果不是promise，把value做为参数传给onFulfilledCallbacks里的每个函数。
6. reject
   • promise的状态是rejected异常是的处理函数
   • 接收 reason 参数，把reason做为参数传给onRejectedCallbacks里的每个函数。
7. 执行 executor，如果有异常，抛给reject
8. 因为Promise是在同步代码执行完成后再执行，所以要把Mypromise的执行方法resolve和reject放在异步队列里

function MyPromise(executor) {
    if (typeof executor !== 'function') {
        throw new TypeError('Promise resolver ' + executor + ' is not a function');
    }
    let self = this;
    this.status = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];
    function resolve(value) {
        if (value instanceof MyPromise) {
            return value.then(resolve, reject);
        }
        if (self.status === 'pending') {
            self.value = value;
            self.status = 'fulfilled';
            self.onFulfilledCallbacks.forEach(item => item(value));
        }
    }
    function reject(reason) {
        if (self.status === 'pending') {
            self.reason = reason;
            self.status = 'rejected';  
            self.onRejectedCallbacks.forEach(item => item(reason));
        }
    }
    try {
        executor(resolve, reject);
    } catch (e) {
        reject(e);
    }
}

MyPromise.prototype.then

1. 作用：接收两个函数参数，第一个函数的参数是 resolve传入的参数，第二个参数是 reject传入的参数。Promise#then
2. onFulfilled
   • MyPromise 成功时执行的方法
   • resolve 的参数会作为value传给 onFulfilled
3. onRejected
   • MyPromise 失败时执行的方法
   • reject 的参数会作为value传给 onRejected
4. 返回一个 MyPromise 实例 newPromise，方便链式调用
5. 对三种状态分别处理
   • 每个状态中创建 newPromise
   • fulfilled
     • 直接执行 onFulfilled，返回值x
     • 把newPromise、x以及newPromise里的resolve、reject做为参数传给 resolutionPromise
     • 把 MyPromise 的参数放在异步队列里
   • rejected
     • 直接执行 onRejected，返回值x
     • 把newPromise、x以及newPromise里的resolve、reject做为参数传给 resolutionPromise
     • 把 MyPromise 的参数放在异步队列里
   • pending
     • 状态待定，把fulfilled和rejected里的异步函数分别加到 onFulfilledCallbacks 和 onRejectedCallbacks的最后一位
6. resolutionPromise 后面细说
7. 用catch捕获异常，执行 reject

MyPromise.prototype.then = function (onFulfilled, onRejected) {
    let self = this;
    typeof onFulfilled !== 'function' && (onFulfilled = function (value) {
        return value;
    });
    typeof onRejected !== 'function' && (onRejected = function (reason) {
        throw reason;
    });
    let newPromise;
    /**
     *  分别处理实例的三种状态
     */
    if (self.status === 'fulfilled') {
        newPromise = new MyPromise(function (resolve, reject) {
            setTimeout(function () {
                try {
                    let x = onFulfilled(self.value);
                    resolutionPromise(newPromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            });
        });
    }
    if (self.status === 'rejected') {
        newPromise = new MyPromise(function (resolve, reject) {
            setTimeout(function () {
                try {
                    let x = onRejected(self.reason);
                    resolutionPromise(newPromise, x, resolve, reject);
                } catch (e) {
                    reject(e);
                }
            });
        });
    }
    if (self.status === 'pending') {
        newPromise = new MyPromise(function (resolve, reject) {
            self.onFulfilledCallbacks.push(function (value) {
                setTimeout(function () {
                    try {
                        let x = onFulfilled(value);
                        resolutionPromise(newPromise, x, resolve, reject);
                    } catch (e) {
                        reject(e);
                    }
                });
            });
            self.onRejectedCallbacks.push(function (reason) {
                setTimeout(function () {
                    try {
                        let x = onRejected(reason);
                        resolutionPromise(newPromise, x, resolve, reject);
                    } catch (e) {
                        reject(e);
                    }
                });
            });
        });
    }
    return newPromise;
};

MyPromise.prototype.catch

1. 作用：捕获异常
2. 返回 MyPromise

MyPromise.prototype.catch = function (onRejected) {
    return this.then(undefined, onRejected);
};

The Promise Resolution Procedure

1. Promise解析过程，是以一个 promise、一个值 x及resolve, reject做为参数的抽象过程
2. promise 等于 x，reject 抛出异常 new TypeError('循环引用')
3. x如果不是对象（不包括 null）或者函数，执行 resolve(x)
4. 获取 x.then 赋值给 then
   • then 如果是 function
     • 把 x做为 this 调用then，第一个参数是 resolvePromise，第二个参数是 rejectPromise
     • resolvePromise和 rejectPromise只有第一次调用有效
     • resolvePromise参数为 y，执行 resolutionPromise(promise, y, resolve, reject)
     • rejectPromise参数为 r，执行 reject(r)
   • then 如果不是 function
     • 执行 resolve(x)
5. 用捕获上一步的异常
   • 执行 reject(e)
   • 如果执行过 resolvePromise或 rejectPromise，忽略

function resolutionPromise(promise, x, resolve, reject) {
    if (promise === x) {
        reject(new TypeError('循环引用'));
    }
    let then, called;
    if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
        try {
            then = x.then;
            if (typeof then === 'function') {
                then.call(x, function (y) {
                    if (called)
                        return;
                    called = true;
                    resolutionPromise(promise, y, resolve, reject);
                }, function (r) {
                    if (called)
                        return;
                    called = true;
                    reject(r);
                })
            } else {
                resolve(x);
            }
        } catch (e) {
            if (called)
                return;
            reject(e);
        }
    } else {
        resolve(x);
    }
}

MyPromise 静态方法

MyPromise.all

• 作用：Promise.all

MyPromise.all = function (promises) {
    let called = false;
    return new MyPromise(function (resolve, reject) {
        let newArr = [], count = 0;
        for (let i = 0; i < promises.length; i++) {
            let item = promises[i];
            if (!(item instanceof MyPromise)) {
                item = MyPromise.resolve(item);
            }
            item.then(function (data) {
                if (!called) {
                    newArr[i] = data;
                    if (i == count) {
                        resolve(newArr);
                        count++;
                    }
                }
            }, function (e) {
                if (!called) {
                    reject(e);
                    called = true;
                }
            });
        }
    });
};

MyPromise.race

• 作用：Promise.race

MyPromise.race = function (promises) {
    return new MyPromise(function (resolve, reject) {
        let called = false;
        for (let i = 0; i < promises.length; i++) {
            let item = promises[i];
            if (!(item instanceof MyPromise)) {
                item = MyPromise.resolve(item);
            }
            item.then(function (data) {
                if (!called) {
                    resolve(data);
                    called = true;
                }
            }, function (e) {
                if (!called) {
                    reject(e);
                    called = true;
                }
            });
        }
    })
};

MyPromise.resolve

• 作用：Promise.resolve

MyPromise.resolve = function (value) {
    if (value instanceof MyPromise) {
        return value;
    }
    return new MyPromise(function (resolve, reject) {
        if (typeof value !== null && typeof value === 'object' && typeof value.then === 'function') {
            value.then();
        } else {
            resolve(value);
        }
    })
};

MyPromise.reject

• 作用：Promise.reject

MyPromise.reject = function (e) {
    return new MyPromise(function (resolve, reject) {
        reject(e);
    })
};

test

• npm i -g promises-aplus-tests
• promises-aplus-tests Promise.js

源码

• Promise.js

参考资料

• Promise/A+规范
• Promise/A+规范(英文)
• JavaScript Promise迷你书(中文版)