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<title>手写promise</title>
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<body>
<script>
class Promise {
constructor(exector) {
this.state = 'pending';
this.value = undefined;
this.reason = undefined;
let resolve = value => {
if (this.state === 'pending') {
this.state = 'fulfilled';
this.value = value;
}
};
let reject = reason => {
if (this.state === 'pending') {
this.state = 'rejected';
this.reason = reason;
}
};
try {
executor(resolve, reject);
} catch (err) {
reject(err)
}
}
then(onFulfilled, onReject) {
if (state === 'fulfilled') {
onFulfilled(this.value)
}
if (state === 'rejected') {
onReject(this.reason)
}
}
}
// promise源码
// 判断变量否为function
const isFunction = variable => typeof variable === 'function'
// 定义Promise的三种状态常量
const PENDING = 'PENDING'
const FULFILLED = 'FULFILLED'
const REJECTED = 'REJECTED'
class MyPromise {
constructor(handle) {
if (!isFunction(handle)) {
throw new Error('MyPromise must accept a function as a parameter')
}
// 添加状态
this._status = PENDING
// 添加状态
this._value = undefined
// 添加成功回调函数队列
this._fulfilledQueues = []
// 添加失败回调函数队列
this._rejectedQueues = []
// 执行handle
try {
handle(this._resolve.bind(this), this._reject.bind(this))
} catch (err) {
this._reject(err)
}
}
// 添加resovle时执行的函数
_resolve(val) {
const run = () => {
if (this._status !== PENDING) return
this._status = FULFILLED
// 依次执行成功队列中的函数，并清空队列
const runFulfilled = (value) => {
let cb;
while (cb = this._fulfilledQueues.shift()) {
cb(value)
}
}
// 依次执行失败队列中的函数，并清空队列
const runRejected = (error) => {
let cb;
while (cb = this._rejectedQueues.shift()) {
cb(error)
}
}
/* 如果resolve的参数为Promise对象，则必须等待该Promise对象状态改变后,
当前Promsie的状态才会改变，且状态取决于参数Promsie对象的状态
*/
if (val instanceof MyPromise) {
val.then(value => {
this._value = value
runFulfilled(value)
}, err => {
this._value = err
runRejected(err)
})
} else {
this._value = val
runFulfilled(val)
}
}
// 为了支持同步的Promise，这里采用异步调用
setTimeout(run, 0)
}
// 添加reject时执行的函数
_reject(err) {
if (this._status !== PENDING) return
// 依次执行失败队列中的函数，并清空队列
const run = () => {
this._status = REJECTED
this._value = err
let cb;
while (cb = this._rejectedQueues.shift()) {
cb(err)
}
}
// 为了支持同步的Promise，这里采用异步调用
setTimeout(run, 0)
}
// 添加then方法
then(onFulfilled, onRejected) {
const {
_value,
_status
} = this
// 返回一个新的Promise对象
return new MyPromise((onFulfilledNext, onRejectedNext) => {
// 封装一个成功时执行的函数
let fulfilled = value => {
try {
if (!isFunction(onFulfilled)) {
onFulfilledNext(value)
} else {
let res = onFulfilled(value);
if (res instanceof MyPromise) {
// 如果当前回调函数返回MyPromise对象，必须等待其状态改变后在执行下一个回调
res.then(onFulfilledNext, onRejectedNext)
} else {
//否则会将返回结果直接作为参数，传入下一个then的回调函数，并立即执行下一个then的回调函数
onFulfilledNext(res)
}
}
} catch (err) {
// 如果函数执行出错，新的Promise对象的状态为失败
onRejectedNext(err)
}
}
// 封装一个失败时执行的函数
let rejected = error => {
try {
if (!isFunction(onRejected)) {
onRejectedNext(error)
} else {
let res = onRejected(error);
if (res instanceof MyPromise) {
// 如果当前回调函数返回MyPromise对象，必须等待其状态改变后在执行下一个回调
res.then(onFulfilledNext, onRejectedNext)
} else {
//否则会将返回结果直接作为参数，传入下一个then的回调函数，并立即执行下一个then的回调函数
onFulfilledNext(res)
}
}
} catch (err) {
// 如果函数执行出错，新的Promise对象的状态为失败
onRejectedNext(err)
}
}
switch (_status) {
// 当状态为pending时，将then方法回调函数加入执行队列等待执行
case PENDING:
this._fulfilledQueues.push(fulfilled)
this._rejectedQueues.push(rejected)
break
// 当状态已经改变时，立即执行对应的回调函数
case FULFILLED:
fulfilled(_value)
break
case REJECTED:
rejected(_value)
break
}
})
}
// 添加catch方法
catch (onRejected) {
return this.then(undefined, onRejected)
}
// 添加静态resolve方法
static resolve(value) {
// 如果参数是MyPromise实例，直接返回这个实例
if (value instanceof MyPromise) return value
return new MyPromise(resolve => resolve(value))
}
// 添加静态reject方法
static reject(value) {
return new MyPromise((resolve, reject) => reject(value))
}
// 添加静态all方法
static all(list) {
return new MyPromise((resolve, reject) => {
/**
* 返回值的集合
*/
let values = []
let count = 0
for (let [i, p] of list.entries()) {
// 数组参数如果不是MyPromise实例，先调用MyPromise.resolve
this.resolve(p).then(res => {
values[i] = res
count++
// 所有状态都变成fulfilled时返回的MyPromise状态就变成fulfilled
if (count === list.length) resolve(values)
}, err => {
// 有一个被rejected时返回的MyPromise状态就变成rejected
reject(err)
})
}
})
}
// 添加静态race方法
static race(list) {
return new MyPromise((resolve, reject) => {
for (let p of list) {
// 只要有一个实例率先改变状态，新的MyPromise的状态就跟着改变
this.resolve(p).then(res => {
resolve(res)
}, err => {
reject(err)
})
}
})
}
finally(cb) {
return this.then(
value => MyPromise.resolve(cb()).then(() => value),
reason => MyPromise.resolve(cb()).then(() => {
throw reason
})
);
}
}
</script>
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</html>