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前言

关于动机，无论是在工作还是面试中，都会遇到Promise的相关使用和原理，手写Promise也有助于学习设计模式以及代码设计。
本文主要介绍了如何使用自己的代码去实现Promise的一些功能。

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以下是本篇文章正文内容

一、前置知识

手写之前，需要知道promise有哪些基本特性：

1、promise有三种状态：pending、fulfilled、rejected。

2、promise的默认状态是pending，状态流转有两种：pending—>fulfilled、pending—>rejected。

3、实例化Promise时，可以接收一个函数作为参数，这个函数有两个参数，分别是resolve、reject方法用于改变实例状态。并能够执行下一个then中的回调函数。
4、then方法中接收两个函数作为参数，第一个函数相当于执行上一个 promise 中执行 resolve 后对应的回调，第二个函数相当于执行上一个 promise 中执行 reject 后对应的回调。

// 1. 实例化 Promise 时
// 可以接收一个函数作为参数，这个函数可以接收到 resolve 和 reject 两个实例方法
// 用于更改当前实例的状态，并把它们接收到的参数传递给下一个 then 对应的参数中new Promise((resolve, reject) => {// resolve 和 reject 可以都执行，但都执行的意义不大，因为 promise 状态发生更改后，就不能在被更改resolve('ok');// reject('err');}).then((value) => {console.log("resolve callback = ", value); // 若执行 resolve，则 value = ok}, (reason) => {console.log("reject callback = ", reason); // 若执行 reject，则 value = err});

5、then操作后会返回一个新的Promise，且支持链式调用。

let promise = new Promise((resolve,reject) => {resolve(11)
})
const a = new Promise((resolve,reject) => {reject('ok')
})
promise.then(res => {console.log(res)return a
}).then(res => console.log(res))

6、Promise.all以数组的形式接收多个Promise，当所有Promise执行完成，且状态都为fulfilled时，返回执行成功的结果；有一个失败，则返回失败的结果。

7、Promise.race以数组的形式接收多个Promise，只要有一个Promise先执行成功，无论什么状态，都返回这个结果，给到下一个then中对应的回调。

二、实现基本功能

1、

• new 操作
• resolve、reject方法
• then方法

// 定义三种状态
const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';class Promise {constructor(executor) {// 1、默认状态 - PENDINGthis.status = PENDING;// 2、内部维护的变量值this.value = undefined;this.reason = undefined;try {executor(this.resolve.bind(this), this.reject.bind(this)) // 实例化传进来的函数会立即执行} catch(err) {this.reject(err)}}resolve(value) {if (this.status === PENDING) {this.value = value;this.status = FULFILLED;}}reject(reason) {if (this.status === PENDING) {this.reason = reason;this.status = REJECTED;}}then(onFulfilled, onRejected) {if (onFulfilled && this.status === FULFILLED) {let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)}if (onRejected && this.status === REJECTED) {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)}        }
}

通过以下代码自测：

new Promise((resolve,reject) => {resolve(1)
}).then(res=> {console.log(res) // 1
})

2、考虑异步的情况。
以上代码都是同步的，Promise实例化时传入了异步函数如setTimeout，在setTimeout中resolve，在下一个then的回调函数获取异步函数的状态呢？

我们可以使用回调队列缓存起来，等待执行：

// 定义三种状态
const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';class Promise {constructor(executor) {// 1、默认状态 - PENDINGthis.status = PENDING;// 2、内部维护的变量值this.value = undefined;this.reason = undefined;this.onResolveCallBack = [];// 缓存 onResolve this.onRejectCallBack = [];// 缓存 onReject try {executor(this.resolve.bind(this), this.reject.bind(this)) // 实例化传进来的函数会立即执行} catch(err) {this.reject(err)}}resolve(value) {if (this.status === PENDING) {this.value = value;this.status = FULFILLED;// 遍历调用 onResolveCallBackthis.onResolveCallBack.forEach(fn => fn());}}reject(reason) {if (this.status === PENDING) {this.reason = reason;this.status = REJECTED;this.onResolveCallBack.forEach(fn => fn());}}then(onFulfilled, onRejected) {if (onFulfilled && this.status === FULFILLED) {let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)}if (onRejected && this.status === REJECTED) {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)}// 当前 promise 状态为 pending，把当前的 onResolve & onReject 缓存起来if (this.status === PENDING) {this.onResolveCallBack.push(() => {onResolve(this.value);});	this.onRejectCallBack.push(() => {onReject(this.value);});}       }
}

用以下代码测试通过：

 let p = new MyPromise((resolve, reject) => {setTimeout(()=>{resolve(1);},1000);}).then(value => {console.log('then resolve = ', value);}, reason => {console.log('then reject = ', reason);});
// then resolve = 1

二、实现链式调用

then(onFulfilled, onRejected) {return new Promise((resolve, reject) => { // 支持链式调用if (onFulfilled && this.status === FULFILLED) {let res = onFulfilled(this.value)resolve(res);}if (onRejected && this.status === REJECTED) {let res = onRejected(this.reason)resolve(res);}if (this.status === PENDING) {this.onResolvedCallbacks.push(() =>{let res = onFulfilled(this.value)resolve(res);})this.onRejectedCallbacks.push(() => {let res = onRejected(this.reason)resolve(res);})}})   }

测试用例：

let promise = new Promise((resolve,reject) => {resolve(1)
})
promise.then(res => {return 2 // 返回普通对象
}).then(res => console.log(res)) // 2

如果在then中返回的是普通对象，上述代码能满足，如果返回的是一个Promise，则还需要补充：

then(onFulfilled, onRejected) {return new Promise((resolve, reject) => { // 支持链式调用if (onFulfilled && this.status === FULFILLED) {let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)}if (onRejected && this.status === REJECTED) {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)}if (this.status === PENDING) {this.onResolvedCallbacks.push(() =>{let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)})this.onRejectedCallbacks.push(() => {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)})}})}resolvePromise(res, resolve, reject) {if(res instanceof Promise) { // 对then中返回Promise做处理res.then(resolve, reject)} else{// 普通值resolve(res)}}

测试用例：

let promise = new Promise((resolve,reject) => {resolve(11)
})
const a = new Promise((resolve,reject) => {reject('ok')
})
promise.then(res => {return a;
}).then(res => console.log(res)) // ok

三、实现Promise.all

由上述前置知识可知，Promise.all是全部成功才返回成功的结果，有一个失败就返回失败的结果：

Promise.all = function(arr) {return new Promise((resolve, reject) => {if (!Array.isArray(arr)) {return reject("参数必须为数组");}let successNum = 0; // 成功的Promise条数let resultArray = [];let totalNum = arr.length; // 总Promise数量let isFail = false;arr.forEach(item => {item.then(res => {successNum++;resultArray.push(res);if (successNum === totalNum) { // 说明是全部成功return resolve(resultArray)}}, err => {if (!isFail) reject(err)isFail = true;})});       })
}

测试用例：

function wait500(input) {return new Promise((resolve, reject) => {setTimeout(()=> {resolve(500)}, 500)})
}
function wait1000(input) {return new Promise((resolve, reject) => {setTimeout(()=> {resolve(1000)}, 1000)})
}
// 全部执行完成之后回调
Promise.all([wait1000(), wait500()]).then(result => {console.log('all end', result)
}, err => {console.log('fafaf', err)
}) // 'all end [500, 1000]'

四、实现Promise.race

由上述前置知识可知，Promise.race是有一个promise先执行成功，无论什么状态，都返回这个结果：

Promise.race = function(arr) {return new Promise((resolve, reject) => {if (!Array.isArray(arr)) {return reject("参数必须为数组");}arr.forEach(item => {item.then(res => {return resolve(res); // 状态不可逆，只要其中一个Promise转变了状态，后续状态就不会发生改变}, err => {reject(err);})});       })}

测试用例：

function wait500(input) {return new Promise((resolve, reject) => {setTimeout(()=> {resolve(500)}, 500)})
}
function wait1000(input) {return new Promise((resolve, reject) => {setTimeout(()=> {resolve(1000)}, 1000)})
}
Promise.race([wait1000(), wait500()]).then(result => {console.log('all end', result)
}, err => {console.log('fafaf', err)
}) // 打印出'all end 500'

总结

总的代码：

const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';class Promise {constructor(executor) {// 1、默认状态 - PENDINGthis.status = PENDING;// 2、内部维护的变量值this.value = undefined;this.reason = undefined;// 存放回调this.onResolvedCallbacks = [];this.onRejectedCallbacks = [];try {executor(this.resolve.bind(this), this.reject.bind(this))} catch(err) {this.reject(err)}}resolve(value) {if (this.status === PENDING) {this.value = value;this.status = FULFILLED;this.onResolvedCallbacks.forEach(fn => fn())}}reject(reason) {if (this.status === PENDING) {this.reason = reason;this.status = REJECTED;this.onRejectedCallbacks.forEach(fn => fn())}}then(onFulfilled, onRejected) {return new Promise((resolve, reject) => { // 支持链式调用if (onFulfilled && this.status === FULFILLED) {let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)}if (onRejected && this.status === REJECTED) {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)}if (this.status === PENDING) {this.onResolvedCallbacks.push(() =>{let res = onFulfilled(this.value)this.resolvePromise(res, resolve, reject)})this.onRejectedCallbacks.push(() => {let res = onRejected(this.reason)this.resolvePromise(res, resolve, reject)})}})}resolvePromise(res, resolve, reject) {if(res instanceof Promise) {res.then(resolve, reject)} else{// 普通值resolve(res)}}
}
// 全部成功才返回成功的结果，有一个失败就返回失败的结果
Promise.all = function(arr) {return new Promise((resolve, reject) => {if (!Array.isArray(arr)) {return reject("参数必须为数组");}let successNum = 0;let resultArray = [];let totalNum = arr.length;let isFail = false;arr.forEach(item => {item.then(res => {successNum++;resultArray.push(res);if (successNum === totalNum) {return resolve(resultArray)}}, err => {if (!isFail) reject(err)isFail = true;})});       })
}// 某个promise先完成，
Promise.race = function(arr) {return new Promise((resolve, reject) => {if (!Array.isArray(arr)) {return reject("参数必须为数组");}arr.forEach(item => {item.then(res => {return resolve(res);}, err => {reject(err);})});       })}