模拟Promise的功能, 按照下面的步骤,一步一步
1. 新建是个构造函数
2. 传入一个可执行函数 函数的入参第一个为 fullFill函数 第二个为 reject函数； 函数立即执行， 参数函数异步执行
3. 状态一旦更改就不可以变更 只能 pending => fulfilled 或者 pending => rejected
4. then 的时候要处理入参的情况 successCallback 和failCallback 均可能为非函数
- 默认的 failCallback 一定要将异常抛出， 这样下一个promise便可将其捕获 异常冒泡的目的
5. then 中执行回调的时候要捕获异常 将其传给下一个promise
- 如果promise状态未变更 则将回调方法添加到对应队列中
- 如果promise状态已经变更 需要异步处理成功或者失败回调
- 因为可能出现 回调结果和当前then返回的Promise一致 从而导致死循环问题
6. catch只是then的一种特殊的写法 方便理解和使用
7. finally 特点
- 不过resolve或者reject都会执行
- 回调没有参数
- 返回一个Promise 且值可以穿透到下一个then或者catch
8. Promise.resolve, Promise.reject 根据其参数返回对应的值 或者状态的Promise即可
9. Proise.all 特点
- 返回一个Promise
- 入参是数组 resolve的情况下出参也是数组 且结果顺序和调用顺序一致
- 所有的值或者promise都完成才能resolve 所有要计数
- 只要有一个为reject 返回的Promise便reject
10. Proise.race 特点
- 返回一个Promise
- 入参是数组 那么出参根据第一个成功或者失败的参数来确定
- 只要有一个resolve 或者reject 便更改返回Promise的状态
const PENDING = 'pending' //等待
const FULFILLED = 'fulfilled' //成功
const REJECTED = 'rejected' //失败
const fulfilledCallback = [] //成功回调函数
const rejectedCallback = [] //失败回调函数
class MyPromise {
constructor (executor) {
try{
executor(this.resolve, this.reject)
} catch (e){
this.reject(e)
}
}
status = PENDING//promise的状态
value = undefined//成功之后的值
reason = undefined//失败之后的值
fulfilledCallback = [] //成功回调函数
rejectedCallback = [] //失败回调函数
resolve = value => {
//如果状态不是等待, 阻止程序继续往下执行
if(this.status !== PENDING){
return
}
this.status = FULFILLED
this.value = value
//判断成功回调是否存在,如果存在, 调用
// this.fulfilledCallback && this.fulfilledCallback(this.value)
while(this.fulfilledCallback.length) this.fulfilledCallback.shift()()
}
reject = reason => {
//如果状态不是等待, 阻止程序继续往下执行
if(this.status !== PENDING){
return
}
this.status = REJECTED
this.reason = reason
//判断失败回调是否存在,如果存在, 调用
// this.rejectedCallback && this.rejectedCallback(this.reason)
while(this.rejectedCallback.length) this.rejectedCallback.shift()()
}
then = (fulfilledCallback, rejectedCallback) => {
fulfilledCallback = fulfilledCallback ? fulfilledCallback : value => value
rejectedCallback = rejectedCallback ? rejectedCallback : reason => {throw reason}
let promise2 = new MyPromise((resolve, reject) => {
//判断状态
if(this.status === FULFILLED){
setTimeout(() => {
try {
//判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
//如果是普通值,直接调用resolve
let x = fulfilledCallback(this.value)
//外面加载完,才能获取到promise2,用异步解决
resolvePromise(promise2,x,resolve, reject)
} catch (error) {
reject(error)
}
},0)
}else if(this.status === REJECTED){
setTimeout(() => {
try {
//判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
//如果是普通值,直接调用resolve
let x = rejectedCallback(this.reason)
//外面加载完,才能获取到promise2,用异步解决
resolvePromise(promise2,x,resolve, reject)
} catch (error) {
reject(error)
}
},0)
}else{
//等待状态
this.fulfilledCallback.push(() => {
setTimeout(() => {
try {
//判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
//如果是普通值,直接调用resolve
let x = fulfilledCallback(this.value)
//外面加载完,才能获取到promise2,用异步解决
resolvePromise(promise2,x,resolve, reject)
} catch (error) {
reject(error)
}
},0)
});
this.rejectedCallback.push(() => {
setTimeout(() => {
try {
//判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
//如果是普通值,直接调用resolve
let x = rejectedCallback(this.reason)
//外面加载完,才能获取到promise2,用异步解决
resolvePromise(promise2,x,resolve, reject)
} catch (error) {
reject(error)
}
},0)
});
}
})
return promise2
}
static all (array) {
let result= []
let index = 0;
return new MyPromise((resolve, reject) => {
function addData (key, value) {
result[key] = value
index++
if (index === array.length) {
resolve(result)
}
}
for (let i = 0; i < array.length; i++) {
if (array[i] instanceof MyPromise) {
//promise对象
array[i].then(value => addData(i, value), reason => reject(reason))
} else {
//普通值, 放到数组里
addData(i, array[i])
}
}
})
}
static resolve (value) {
if(value instanceof MyPromise) return value
return new MyPromise(resolve => resolve(value))
}
finally (callback) {
return this.then(value => {
return MyPromise.resolve(callback()).then(() => value)
}, reason => {
return MyPromise.resolve(callback()).then(() => {throw reason})
})
}
catch (rejectedCallback) {
return this.then(undefined, rejectedCallback)
}
}
function resolvePromise(promise2,x,resolve, reject){
if(x === promise2){
return reject(new TypeError('啦啦啦啦'))
}
if(x instanceof MyPromise){
//是promise
//往下直接传
x.then(resolve,reject)
}else{
//普通值
resolve(x)
}
}
module.exports = MyPromise;
标签：功能,resolve,rejectedCallback,value,reason,Promise,reject,promise,模拟 From： https://www.cnblogs.com/Young111/p/16940017.html