从零开始实现一个 Promise

2022-10-14
浙江
本文字数：5276 字
阅读完需：约 17 分钟

1.Promise 产生背景及规范

众所周知，Promise 是 ES6 引入的新特性，旨在解决回调地狱。下面是一个简单的例子：控制接口调用顺序：

apiA-->apiB-->apiC。复杂的业务，开发人员会裂开。后生在此向老前辈致敬。

// 回调地狱apiA({    handleSuccess(resA){        apiB({            handleSuccess(resB){                apiC({                    handleSuccess(resC){
                    }                })            }        })    }})

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因此 Promise/A+规范应运而生，ES6 的 Promise 就是遵循规范开发出来的。

2. 同步 Promise

阅读规范可得下面几点基本要求:

Promise 存在三个状态：pending（等待态）、fulfilled（成功态）、rejected（失败态）
pending 为初始态，并可以转化为 fulfilled 和 rejected
成功时，不可转为其他状态，且必须有一个不可改变的值（value）
失败时，不可转为其他状态，且必须有一个不可改变的原因（reason）
new Promise(executor=(resolve,reject)=>{resolve(value)})，resolve(value)将状态置为 fulfilled
new Promise(executor=(resolve,reject)=>{reject(reson)})，reject(reson)将状态置为 rejected
若是 executor 运行异常执行 reject()
thenable：then(onFulfilled, onRejected)
onFulfilled：status 为 fulfilled，执行 onFulfilled，传入 value
onRejected：status 为 rejected，执行 onRejected，传入 reason

// 1.Promise存在三个状态：pending（等待态）、fulfilled（成功态）、rejected（失败态）const STATUS_PENDING = 'pending'const STATUS_FULFILLED = 'fulfilled'const STATUS_REJECTED = 'rejected'class myPromise {  constructor(executor) {      // pending为初始态，并可以转化为fulfilled和rejected    this.status = STATUS_PENDING    this.value = '' // 3    this.reason = '' // 4
    let resolve = value => {    // 5.      if (this.status === STATUS_PENDING) {        this.status = STATUS_FULFILLED        this.value = value      }    }    let reject = reason => {    //6.      if (this.status === STATUS_PENDING) {        this.status = STATUS_REJECTED        this.reason = reason      }    }    // 7.    try {      executor(resolve, reject);    } catch (err) {      reject(err);    }  }  // 8.  then(onFulfilled = () => {}, onRejected = () => {}) {      // 8.1    if (this.status === STATUS_FULFILLED) {      onFulfilled(this.value)    }    // 8.2    if (this.status === STATUS_REJECTED) {      onRejected(this.reason)    }  }}new myPromise(resolve => {  console.log('before resolve')  resolve(1)}).then(res => {  console.log(res)})
new myPromise((resolve, reject) => {  console.log('before reject')  reject('reject error')}).then(res => {  console.log(res)}, error => {  console.log(error)})

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3. 异步 Promise

new myPromise(resolve => {  console.log('before resolve')  setTimeout(()=>{      resolve(1)  },1000)}).then(res => {  console.log(res)})

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promise 的状态只能在 resolve 或者 reject 的时候改变，同步代码执行到 then 回调的时候 promise 的状态还是 pending，明细不符合我们的期望。

如果换做是你，你会怎么解决这个问题？举个栗子，你正在处理一堆事情（pending 状态），然后（then），老板曰: 一秒内做完手上的事来一下我办公室，做不完滚蛋。你怕忘记，一般会用清单记录 onResolvedCallbacks = ['做完了手上的事,去老板办公室']，onRejectedCallbacks = ['做不完,滚蛋']，1 秒后看完成结果，再依据选择下一步。

完善后的代码如下：参考前端手写面试题详细解答

const STATUS_PENDING = 'pending'const STATUS_FULFILLED = 'fulfilled'const STATUS_REJECTED = 'rejected'class myPromise {  constructor(executor) {    this.status = STATUS_PENDING    this.value = ''    this.reason = ''
    // 成功存放的数组    this.onResolvedCallbacks = [];    // 失败存放法数组    this.onRejectedCallbacks = [];
    let resolve = value => {      if (this.status === STATUS_PENDING) {        this.status = STATUS_FULFILLED        this.value = value        // pending->fulfilled 按照成功清单执行        this.onResolvedCallbacks.forEach(fn => fn())      }    }    let reject = reason => {      if (this.status === STATUS_PENDING) {        this.status = STATUS_REJECTED        this.reason = reason        // pending->rejected 按照异常清单执行        this.onRejectedCallbacks.forEach(fn => fn());      }    }    try {      executor(resolve, reject);    } catch (err) {      reject(err);    }  }  then(onFulfilled = () => {}, onRejected = () => {}) {    if (this.status === STATUS_FULFILLED) {      onFulfilled(this.value)    }    if (this.status === STATUS_REJECTED) {      onRejected(this.reason)    }    // 忙碌状态,先记录老板吩咐的内容    if (this.status === STATUS_PENDING) {      // onFulfilled传入到成功数组      this.onResolvedCallbacks.push(() => onFulfilled(this.value))      // onRejected传入到失败数组      this.onRejectedCallbacks.push(() => onRejected(this.reason))    }  }}// 异步new myPromise((resolve, reject) => {  console.log('老板曰: 一秒做完手上的事来一下我办公室,做不完滚蛋')  setTimeout(() => {    if (false) { // 臣妾做不到啊      resolve('做完了手上的事,去老板办公室')    } else {      reject('做不完,滚蛋')    }  }, 1000)}).then(res => {  console.log(`1s 后:${res}`)}, error => {  console.log(`1s 后:${error}`)})// 老板曰: 一秒做完手上的事来一下我办公室,做不完滚蛋// 1s 后:做不完,滚蛋

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4. new Promise().then().then()...

这个思路倒是挺简单，就是 then 函数返回值为另一个 Promise 实例。根据规范修改后如下：

const PENDING = 'pending'const FULFILLED = 'fulfilled'const REJECTED = 'rejected'
function resolvePromise(promise2, x, resolve, reject) {  // 循环引用报错  if (x === promise2) {    // reject报错    return reject(new TypeError('Chaining cycle detected for promise'));  }  // 防止多次调用  let called;  // x不是null 且x是对象或者函数  if (x != null && (typeof x === 'object' || typeof x === 'function')) {    try {      // A+规定，声明then = x的then方法      let then = x.then;      // 如果then是函数，就默认是promise了      if (typeof then === 'function') {        // 就让then执行 第一个参数是this   后面是成功的回调 和 失败的回调        then.call(x, y => {          // 成功和失败只能调用一个          if (called) return;          called = true;          // resolve的结果依旧是promise 那就继续解析          resolvePromise(promise2, y, resolve, reject);        }, err => {          // 成功和失败只能调用一个          if (called) return;          called = true;          reject(err); // 失败了就失败了        })      } else {        resolve(x); // 直接成功即可      }    } catch (e) {      // 也属于失败      if (called) return;      called = true;      // 取then出错了那就不要在继续执行了      reject(e);    }  } else {    resolve(x);  }}
class Promise {  constructor(executor) {    this.state = PENDING    this.value = ''    this.reason = ''
    // 成功存放的数组    this.onResolvedCallbacks = [];    // 失败存放法数组    this.onRejectedCallbacks = [];
    let resolve = value => {      if (this.state === PENDING) {        this.state = FULFILLED        this.value = value        // pending->fulfilled 按照成功清单执行        this.onResolvedCallbacks.forEach(fn => fn())      }    }    let reject = reason => {      if (this.state === PENDING) {        this.state = REJECTED        this.reason = reason        // pending->rejected 按照异常清单执行        this.onRejectedCallbacks.forEach(fn => fn());      }    }    try {      executor(resolve, reject);    } catch (err) {      reject(err);    }  }  then(onFulfilled, onRejected) {    // onFulfilled如果不是函数，就忽略onFulfilled，直接返回value    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;    // onRejected如果不是函数，就忽略onRejected，扔出错误    onRejected = typeof onRejected === 'function' ? onRejected : err => {      throw err    };    let promise2 = new Promise((resolve, reject) => {      if (this.state === FULFILLED) {        // 异步解决：        // onRejected返回一个普通的值，失败时如果直接等于 value => value，        // 则会跑到下一个then中的onFulfilled中，        setTimeout(() => {          try {            let x = onFulfilled(this.value);            resolvePromise(promise2, x, resolve, reject);          } catch (e) {            reject(e);          }        }, 0);      };      if (this.state === REJECTED) {        setTimeout(() => {          try {            let x = onRejected(this.reason);            resolvePromise(promise2, x, resolve, reject);          } catch (e) {            reject(e);          }        }, 0);      };      if (this.state === PENDING) {        this.onResolvedCallbacks.push(() => {          setTimeout(() => {            try {              let x = onFulfilled(this.value);              resolvePromise(promise2, x, resolve, reject);            } catch (e) {              reject(e);            }          }, 0);        });        this.onRejectedCallbacks.push(() => {          setTimeout(() => {            try {              let x = onRejected(this.reason);              resolvePromise(promise2, x, resolve, reject);            } catch (e) {              reject(e);            }          }, 0)        });      };    });    return promise2;  }}new Promise(resolve => {    console.log(0)    setTimeout(() => resolve(1), 3000)  })  .then(res => {    console.log(res)    return new Promise(resolve => {      console.log(2)      setTimeout(() => {        resolve(3)      }, 3000)    })  })  .then(res => {    console.log(res)  })

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5. catch、resolve、reject、race 和 all

1. catch(特殊的 then 方法)

catch(fn){    return this.then(null,fn)}

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2. resolve(resolve 一个值)

Promise.resolve = val => new Promise(resolve=> resolve(val))

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3. reject(reject 一个值)

Promise.reject = val => new Promise((resolve,reject)=> reject(val))

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4. race

Promise.race([p1, p2, p3])里面哪个结果获得的快，就返回那个结果，不管结果本身是成功状态还是失败状态。

Promise.race = promises =>  new Promise((resolve, reject) =>    promises.forEach(pro => pro.then(resolve, reject))  )

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5. all

Promise.all 可以将多个 Promise 实例包装成一个新的 Promise 实例。同时，成功和失败的返回值是不同的，成功的时候返回的是一个结果数组，而失败的时候则返回最先被 reject 失败状态的值。

Promise.all = function (promises) {  return new Promise((resolve, reject) => {    let index = 0;    let result = [];    if (promises.length === 0) {      resolve(result);    } else {      function processValue(i, data) {        result[i] = data;        if (++index === promises.length) {          resolve(result);        }      }      for (let i = 0; i < promises.length; i++) {        //promises[i] 可能是普通值        Promise.resolve(promises[i]).then((data) => {          processValue(i, data);        }, (err) => {          reject(err);          return;        });      }    }  });}

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本文旨在记录学习过程，通过手打代码加深印象

发布于: 刚刚阅读数: 4

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从零开始实现一个 Promise

1.Promise 产生背景及规范

2. 同步 Promise

3. 异步 Promise

4. new Promise().then().then()...

5. catch、resolve、reject、race 和 all

1. catch(特殊的 then 方法)

2. resolve(resolve 一个值)

3. reject(reject 一个值)

4. race

5. all

helloworld1024fd

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