实现 Promise
// 模拟实现Promise
// Promise利用三大手段解决回调地狱：
// 1. 回调函数延迟绑定
// 2. 返回值穿透
// 3. 错误冒泡
// 定义三种状态
const PENDING = 'PENDING'; // 进行中
const FULFILLED = 'FULFILLED'; // 已成功
const REJECTED = 'REJECTED'; // 已失败
class Promise {
constructor(exector) {
// 初始化状态
this.status = PENDING;
// 将成功、失败结果放在this上，便于then、catch访问
this.value = undefined;
this.reason = undefined;
// 成功态回调函数队列
this.onFulfilledCallbacks = [];
// 失败态回调函数队列
this.onRejectedCallbacks = [];
const resolve = value => {
// 只有进行中状态才能更改状态
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
// 成功态函数依次执行
this.onFulfilledCallbacks.forEach(fn => fn(this.value));
}
}
const reject = reason => {
// 只有进行中状态才能更改状态
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
// 失败态函数依次执行
this.onRejectedCallbacks.forEach(fn => fn(this.reason))
}
}
try {
// 立即执行executor
// 把内部的resolve和reject传入executor，用户可调用resolve和reject
exector(resolve, reject);
} catch(e) {
// executor执行出错，将错误内容reject抛出去
reject(e);
}
}
then(onFulfilled, onRejected) {
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
onRejected = typeof onRejected === 'function'? onRejected :
reason => { throw new Error(reason instanceof Error ? reason.message : reason) }
// 保存this
const self = this;
return new Promise((resolve, reject) => {
if (self.status === PENDING) {
self.onFulfilledCallbacks.push(() => {
// try捕获错误
try {
// 模拟微任务
setTimeout(() => {
const result = onFulfilled(self.value);
// 分两种情况：
// 1. 回调函数返回值是Promise，执行then操作
// 2. 如果不是Promise，调用新Promise的resolve函数
result instanceof Promise ? result.then(resolve, reject) : resolve(result);
})
} catch(e) {
reject(e);
}
});
self.onRejectedCallbacks.push(() => {
// 以下同理
try {
setTimeout(() => {
const result = onRejected(self.reason);
// 不同点：此时是reject
result instanceof Promise ? result.then(resolve, reject) : resolve(result);
})
} catch(e) {
reject(e);
}
})
} else if (self.status === FULFILLED) {
try {
setTimeout(() => {
const result = onFulfilled(self.value);
result instanceof Promise ? result.then(resolve, reject) : resolve(result);
});
} catch(e) {
reject(e);
}
} else if (self.status === REJECTED) {
try {
setTimeout(() => {
const result = onRejected(self.reason);
result instanceof Promise ? result.then(resolve, reject) : resolve(result);
})
} catch(e) {
reject(e);
}
}
});
}
catch(onRejected) {
return this.then(null, onRejected);
}
static resolve(value) {
if (value instanceof Promise) {
// 如果是Promise实例，直接返回
return value;
} else {
// 如果不是Promise实例，返回一个新的Promise对象，状态为FULFILLED
return new Promise((resolve, reject) => resolve(value));
}
}
static reject(reason) {
return new Promise((resolve, reject) => {
reject(reason);
})
}
static all(promiseArr) {
const len = promiseArr.length;
const values = new Array(len);
// 记录已经成功执行的promise个数
let count = 0;
return new Promise((resolve, reject) => {
for (let i = 0; i < len; i++) {
// Promise.resolve()处理，确保每一个都是promise实例
Promise.resolve(promiseArr[i]).then(
val => {
values[i] = val;
count++;
// 如果全部执行完，返回promise的状态就可以改变了
if (count === len) resolve(values);
},
err => reject(err),
);
}
})
}
static race(promiseArr) {
return new Promise((resolve, reject) => {
promiseArr.forEach(p => {
Promise.resolve(p).then(
val => resolve(val),
err => reject(err),
)
})
})
}
}
