請手寫代碼實現一個promise

第一步：promise的聲明

class Promise{
  // 構造器
  constructor(executor){
    // 成功
    let resolve = () => { };
    // 失敗
    let reject = () => { };
    // 當即執行
    executor(resolve, reject);
  }
}

第二步：三個基本狀態（pending、fulfilled、rejected）

class Promise{
  constructor(executor){
    // 初始化state爲等待態
    this.state = 'pending';
    // 成功的值
    this.value = undefined;
    // 失敗的緣由
    this.reason = undefined;
    let resolve = value => {
      // state改變,resolve調用就會失敗
      if (this.state === 'pending') {
        // resolve調用後，state轉化爲成功態
        this.state = 'fulfilled';
        // 儲存成功的值
        this.value = value;
      }
    };
    let reject = reason => {
      // state改變,reject調用就會失敗
      if (this.state === 'pending') {
        // reject調用後，state轉化爲失敗態
        this.state = 'rejected';
        // 儲存失敗的緣由
        this.reason = reason;
      }
    };
    // 若是executor執行報錯，直接執行reject
    try{
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
}

第三步：then方法（兩個參數：onFulfilled,onRejected）

onFulfilled,onRejected若是他們是函數，

則必須分別在fulfilled，rejected後被調用，value或reason依次做爲他們的第一個參數

class Promise{
  constructor(executor){...}
  // then 方法 有兩個參數onFulfilled onRejected
  then(onFulfilled,onRejected) {
    // 狀態爲fulfilled，執行onFulfilled，傳入成功的值
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    };
    // 狀態爲rejected，執行onRejected，傳入失敗的緣由
    if (this.state === 'rejected') {
      onRejected(this.reason);
    };
  }
}

第四步：異步的實現

當resolve在setTomeout內執行，then時state仍是pending等待狀態

咱們就須要在then調用的時候，將成功和失敗存到各自的數組，一旦reject或者resolve，就調用它們

class Promise{
  constructor(executor){
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    // 成功存放的數組
    this.onResolvedCallbacks = [];
    // 失敗存放法數組
    this.onRejectedCallbacks = [];
    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        // 一旦resolve執行，調用成功數組的函數
        this.onResolvedCallbacks.forEach(fn=>fn());
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        // 一旦reject執行，調用失敗數組的函數
        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);
    };
    // 當狀態state爲pending時
    if (this.state === 'pending') {
      // onFulfilled傳入到成功數組
      this.onResolvedCallbacks.push(()=>{
        onFulfilled(this.value);
      })
      // onRejected傳入到失敗數組
      this.onRejectedCallbacks.push(()=>{
        onRejected(this.reason);
      })
    }
  }
}

第五步：鏈式調用

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

這就是鏈式調用，用來解決回調地獄

一、爲了達成鏈式，咱們默認在第一個then裏返回一個promise，叫promise2，將它傳遞下一個then中。

二、咱們須要在then中return一個參數，這個就叫x，x若是是promise，則取它的結果，做爲promise2成功的結果。若是是普通值，直接做爲promise2成功的結果，判斷x的函數叫resolvePromise，帶四個參數（promise2，x，resolve，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) {
    // 聲明返回的promise2
    let promise2 = new Promise((resolve, reject)=>{
      if (this.state === 'fulfilled') {
        let x = onFulfilled(this.value);
        // resolvePromise函數，處理本身return的promise和默認的promise2的關係
        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，完成鏈式
    return promise2;
  }
}

第六步：實現resolvePromise函數

function resolvePromise(promise2, x, resolve, reject){
  // 循環引用報錯
  if(x === promise2){
    // reject報錯（檢測到promise的連接循環）

    return reject(new TypeError('Chaining cycle detected for promise'));
  }
  // 防止屢次調用
  let called;
  // x不是null 且x是對象或者函數
  if (x != null && (typeof x === 'object' || typeof x === 'function')) {
    try {
      // A+規定，聲明then = x的then方法
      let then = x.then;
      // 若是then是函數，就默認是promise了
      if (typeof then === 'function') { 
        // 就讓then執行 第一個參數是this   後面是成功的回調 和 失敗的回調
        then.call(x, y => {
          // 成功和失敗只能調用一個
          if (called) return;
          called = true;
          // resolve的結果依舊是promise 那就繼續解析
          resolvePromise(promise2, y, resolve, reject);
        }, err => {
          // 成功和失敗只能調用一個
          if (called) return;
          called = true;
          reject(err);// 失敗了就失敗了
        })
      } else {
        resolve(x); // 直接成功便可
      }
    } catch (e) {
      // 也屬於失敗
      if (called) return;
      called = true;
      // 取then出錯了那就不要在繼續執行了
      reject(e); 
    }
  } else {
    resolve(x);
  }
}

第七步：解決一些問題

一、onFulfilled或onRejected不能同步被調用，必須異步調用。咱們就用setTimeout解決異步問題

二、onFulfilled返回一個普通的值，成功時直接等於 value => value

三、onRejected返回一個普通的值，失敗時若是直接等於 value => value，則會跑到下一個then中的onFulfilled中，因此直接扔出一個錯誤 reason => throw err

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若是不是函數，就忽略onFulfilled，直接返回value
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
    // onRejected若是不是函數，就忽略onRejected，直接扔出錯誤
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
    let promise2 = new Promise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        // 異步
        setTimeout(() => {
          try {
            let x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      };
      if (this.state === 'rejected') {
        // 異步
        setTimeout(() => {
          // 若是報錯
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      };
      if (this.state === 'pending') {
        this.onResolvedCallbacks.push(() => {
          // 異步
          setTimeout(() => {
            try {
              let x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0);
        });
        this.onRejectedCallbacks.push(() => {
          // 異步
          setTimeout(() => {
            try {
              let x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0)
        });
      };
    });
    // 返回promise，完成鏈式
    return promise2;
  }
}

 

祝賀：一個promise就在本身筆下誕生啦~~

 

參考資料：

https://github.com/xieranmaya/blog/issues/3