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
作用：创建
MyPromise实例。PromiseMyPromise接收一个回掉函数
executor-
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
-
-
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。
-
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。
-
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。
执行
executor，如果有异常，抛给reject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
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-
onFulfilledMyPromise成功时执行的方法resolve的参数会作为value传给onFulfilled
-
onRejectedMyPromise失败时执行的方法reject的参数会作为value传给onRejected
返回一个
MyPromise实例newPromise，方便链式调用-
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
- 状态待定，把
resolutionPromise后面细说用
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
作用：捕获异常
返回
MyPromise
MyPromise.prototype.catch = function (onRejected) {
return this.then(undefined, onRejected);
};
The Promise Resolution Procedure
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promise等于x，reject抛出异常new TypeError('循环引用')x如果不是对象（不包括null）或者函数，执行resolve(x)-
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rejectPromiseresolvePromise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)
- 执行
-
-
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)