BAT前端经典面试问题：史上最最最详细的手写Promise教程

我们工作中免不了运用promise用来解决异步回调问题。平时用的很多库或者插件都运用了promise 例如axios、fetch等等。但是你知道promise是咋写出来的呢？

别怕～这里有本promisesA+规范，便宜点10元卖给你了。

1、Promise 的声明

首先呢，promise肯定是一个类，我们就用class来声明。

由于new Promise((resolve, reject)=>{})，所以传入一个参数（函数），秘籍里叫他executor，传入就执行。
executor里面有两个参数，一个叫resolve（成功），一个叫reject（失败）。
由于resolve和reject可执行，所以都是函数，我们用let声明。

class Promise{
  // 构造器
  constructor(executor){
    // 成功
    let resolve = () => { };
    // 失败
    let reject = () => { };
    // 立即执行
    executor(resolve, reject);
  }
}

解决基本状态

秘籍对Promise有规定：

Promise存在三个状态（state）pending、fulfilled、rejected
pending（等待态）为初始态，并可以转化为fulfilled（成功态）和rejected（失败态）
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((resolve, reject)=>{resolve(value)}) resolve为成功，接收参数value，状态改变为fulfilled，不可再次改变。
new Promise((resolve, reject)=>{reject(reason)}) reject为失败，接收参数reason，状态改变为rejected，不可再次改变。
若是executor函数报错直接执行reject();

于是乎，我们获得以下代码

class Promise{
  constructor(executor){
    // 初始化state为等待态
    this.state = 'pending';
    // 成功的值
    this.value = undefined;
    // 失败的原因
    this.reason = undefined;
    let resolve = value => {
      // state改变,resolve调用就会失败
      if (this.state === 'pending') {
        // resolve调用后，state转化为成功态
        this.state = 'fulfilled';
        // 储存成功的值
        this.value = value;
      }
    };
    let reject = reason => {
      // state改变,reject调用就会失败
      if (this.state === 'pending') {
        // reject调用后，state转化为失败态
        this.state = 'rejected';
        // 储存失败的原因
        this.reason = reason;
      }
    };
    // 如果executor执行报错，直接执行reject
    try{
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
}

then方法

秘籍规定:Promise有一个叫做then的方法，里面有两个参数：onFulfilled,onRejected,成功有成功的值，失败有失败的原因

当状态state为fulfilled，则执行onFulfilled，传入this.value。当状态state为rejected，则执行onRejected，传入this.reason
onFulfilled,onRejected如果他们是函数，则必须分别在fulfilled，rejected后被调用，value或reason依次作为他们的第一个参数

class Promise{
  constructor(executor){...}
  // then 方法 有两个参数onFulfilled onRejected
  then(onFulfilled,onRejected) {
    // 状态为fulfilled，执行onFulfilled，传入成功的值
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    };
    // 状态为rejected，执行onRejected，传入失败的原因
    if (this.state === 'rejected') {
      onRejected(this.reason);
    };
  }
}

这下武学初成，可以对付对付江湖小杂毛了，但是对于带setTimeout的江洋大盗还是没辙。

解决异步实现

现在基本可以实现简单的同步代码，但是当resolve在setTomeout内执行，then时state还是pending等待状态我们就需要在then调用的时候，将成功和失败存到各自的数组，一旦reject或者resolve，就调用它们

类似于发布订阅，先将then里面的两个函数储存起来，由于一个promise可以有多个then，所以存在同一个数组内。

// 多个then的情况
let p = new Promise();
p.then();
p.then();

成功或者失败时，forEach调用它们

class Promise{
  constructor(executor){
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    // 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;
        // 一旦resolve执行，调用成功数组的函数
        this.onResolvedCallbacks.forEach(fn=>fn());
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        // 一旦reject执行，调用失败数组的函数
        this.onRejectedCallbacks.forEach(fn=>fn());
      }
    };
    try{
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled,onRejected) {
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    };
    if (this.state === 'rejected') {
      onRejected(this.reason);
    };
    // 当状态state为pending时
    if (this.state === '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);
      })
    }
  }
}

解决链式调用

我门常常用到new Promise().then().then(),这就是链式调用，用来解决回调地狱

1、为了达成链式，我们默认在第一个then里返回一个promise。秘籍规定了一种方法，就是在then里面返回一个新的promise,称为promise2：promise2 = new Promise((resolve, reject)=>{})

将这个promise2返回的值传递到下一个then中
如果返回一个普通的值，则将普通的值传递给下一个then中

2、当我们在第一个then中return了一个参数（参数未知，需判断）。这个return出来的新的promise就是onFulfilled()或onRejected()的值

秘籍则规定onFulfilled()或onRejected()的值，即第一个then返回的值，叫做x，判断x的函数叫做resolvePromise

首先，要看x是不是promise。
如果是promise，则取它的结果，作为新的promise2成功的结果
如果是普通值，直接作为promise2成功的结果
所以要比较x和promise2
resolvePromise的参数有promise2（默认返回的promise）、x（我们自己return的对象）、resolve、reject
resolve和reject是promise2的

class Promise{
  constructor(executor){
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onResolvedCallbacks.forEach(fn=>fn());
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(fn=>fn());
      }
    };
    try{
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled,onRejected) {
    // 声明返回的promise2
    let promise2 = new Promise((resolve, reject)=>{
      if (this.state === 'fulfilled') {
        let x = onFulfilled(this.value);
        // resolvePromise函数，处理自己return的promise和默认的promise2的关系
        resolvePromise(promise2, x, resolve, reject);
      };
      if (this.state === 'rejected') {
        let x = onRejected(this.reason);
        resolvePromise(promise2, x, resolve, reject);
      };
      if (this.state === 'pending') {
        this.onResolvedCallbacks.push(()=>{
          let x = onFulfilled(this.value);
          resolvePromise(promise2, x, resolve, reject);
        })
        this.onRejectedCallbacks.push(()=>{
          let x = onRejected(this.reason);
          resolvePromise(promise2, x, resolve, reject);
        })
      }
    });
    // 返回promise，完成链式
    return promise2;
  }
}

完成resolvePromise函数

秘籍规定了一段代码，让不同的promise代码互相套用，叫做resolvePromise

如果 x === promise2，则是会造成循环引用，自己等待自己完成，则报“循环引用”错误

let p = new Promise(resolve => {
  resolve(0);
});
var p2 = p.then(data => {
  // 循环引用，自己等待自己完成，一辈子完不成
  return p2;
})

1、判断x

Otherwise, if x is an object or function,Let then be x.then
x 不能是null
x 是普通值直接resolve(x)
x 是对象或者函数（包括promise），let then = x.then 2、当x是对象或者函数（默认promise）
声明了then
如果取then报错，则走reject()
如果then是个函数，则用call执行then，第一个参数是this，后面是成功的回调和失败的回调
如果成功的回调还是pormise，就递归继续解析 3、成功和失败只能调用一个所以设定一个called来防止多次调用

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);
  }
}

解决其他问题

1、秘籍规定onFulfilled,onRejected都是可选参数，如果他们不是函数，必须被忽略

onFulfilled返回一个普通的值，成功时直接等于 value => value
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，所以直接扔出一个错误reason => throw err 2、秘籍规定onFulfilled或onRejected不能同步被调用，必须异步调用。我们就用setTimeout解决异步问题
如果onFulfilled或onRejected报错，则直接返回reject()

class Promise{
  constructor(executor){
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onResolvedCallbacks.forEach(fn=>fn());
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        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，直接扔出错误
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
    let promise2 = new Promise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        // 异步
        setTimeout(() => {
          try {
            let x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      };
      if (this.state === 'rejected') {
        // 异步
        setTimeout(() => {
          // 如果报错
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      };
      if (this.state === 'pending') {
        this.onResolvedCallbacks.push(() => {
          // 异步
          setTimeout(() => {
            try {
              let x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0);
        });
        this.onRejectedCallbacks.push(() => {
          // 异步
          setTimeout(() => {
            try {
              let x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0)
        });
      };
    });
    // 返回promise，完成链式
    return promise2;
  }
}

大功告成

顺便附赠catch和resolve、reject、race、all方法

class Promise{
  constructor(executor){
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = [];
    this.onRejectedCallbacks = [];
    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onResolvedCallbacks.forEach(fn=>fn());
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(fn=>fn());
      }
    };
    try{
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled,onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
    let promise2 = new Promise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        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;
  }
  catch(fn){
    return this.then(null,fn);
  }
}
function resolvePromise(promise2, x, resolve, reject){
  if(x === promise2){
    return reject(new TypeError('Chaining cycle detected for promise'));
  }
  let called;
  if (x != null && (typeof x === 'object' || typeof x === 'function')) {
    try {
      let then = x.then;
      if (typeof then === 'function') { 
        then.call(x, y => {
          if(called)return;
          called = true;
          resolvePromise(promise2, y, resolve, reject);
        }, err => {
          if(called)return;
          called = true;
          reject(err);
        })
      } else {
        resolve(x);
      }
    } catch (e) {
      if(called)return;
      called = true;
      reject(e); 
    }
  } else {
    resolve(x);
  }
}
//resolve方法
Promise.resolve = function(val){
  return new Promise((resolve,reject)=>{
    resolve(val)
  });
}
//reject方法
Promise.reject = function(val){
  return new Promise((resolve,reject)=>{
    reject(val)
  });
}
//race方法 
Promise.race = function(promises){
  return new Promise((resolve,reject)=>{
    for(let i=0;i<promises.length;i++){
      promises[i].then(resolve,reject)
    };
  })
}
//all方法(获取所有的promise，都执行then，把结果放到数组，一起返回)
Promise.all = function(promises){
  let arr = [];
  let i = 0;
  function processData(index,data){
    arr[index] = data;
    i++;
    if(i == promises.length){
      resolve(arr);
    };
  };
  return new Promise((resolve,reject)=>{
    for(let i=0;i<promises.length;i++){
      promises[i].then(data=>{
        processData(i,data);
      },reject);
    };
  });
}

如何验证我们的promise是否正确

1、先在后面加上下述代码

2、npm 有一个promises-aplus-tests插件 npm i promises-aplus-tests -g 可以全局安装 mac用户最前面加上sudo

3、命令行 promises-aplus-tests [js文件名] 即可验证

// 目前是通过他测试 他会测试一个对象
// 语法糖
Promise.defer = Promise.deferred = function () {
  let dfd = {}
  dfd.promise = new Promise((resolve,reject)=>{
    dfd.resolve = resolve;
    dfd.reject = reject;
  });
  return dfd;
}
module.exports = Promise;
//npm install promises-aplus-tests 用来测试自己的promise 符不符合promisesA+规范