1. 初始结构
    2. this指向
    3. 执行异常
    4. 异步
    5. 回调保存
    1. console.log("promise");
    3. class myPromise {
    4.   static PENDING = "待定";
    5.   static FULFILLED = "成功";
    6.   static REJECTED = "拒绝";
    7.   constructor(func) {
    8.     this.status = myPromise.PENDING
    9.     this.result = null
    10.     this.resolveCallbacks = [] // 待定状态下保存回调函数
    11.     this.rejectCallbacks = []
    12.     try {
    13.       // 修改this指向
    14.       func(this.resolve.bind(this), this.reject.bind(this))
    15.     } catch (err) {
    16.       // 执行异常处理
    17.       console.log(err);
    18.       this.reject(err)
    19.     }
    20.   }
    21.   resolve(result) {
    22.     // 延时函数模拟异步，执行处理后的回调
    23.     setTimeout(() => {
    24.       if (this.status === myPromise.PENDING) {
    25.         this.status = myPromise.FULFILLED
    26.         this.result = result
    27.         this.resolveCallbacks.forEach(cb => {
    28.           cb(result)
    29.         })
    30.       }
    31.     })
    32.   }
    33.   reject(result) {
    34.     setTimeout(() => {
    35.       if (this.status === myPromise.PENDING) {
    36.         this.status = myPromise.REJECTED
    37.         this.result = result
    38.         this.rejectCallbacks.forEach(cb => {
    39.           cb(result)
    40.         })
    41.       }
    42.     })
    43.   }
    44.   // then方法
    45.   then(onFULFILED, onREJECTED) {
    46.     return new myPromise((resolve, reject) => {
    47.       onFULFILED = typeof onFULFILED === 'function' ? onFULFILED : () => { }
    48.       onREJECTED = typeof onREJECTED === 'function' ? onFULFILED : () => { }
    49.       if (this.status === myPromise.PENDING) {
    50.         // 待定状态下，回调保存
    51.         this.resolveCallbacks.push(onFULFILED)
    52.         this.rejectCallbacks.push(onREJECTED)
    53.       }
    54.       if (this.status === myPromise.FULFILLED) {
    55.         // 模拟异步处理
    56.         setTimeout(() => {
    57.           onFULFILED(this.result)
    58.         })
    59.       }
    60.       if (this.status === myPromise.REJECTED) {
    61.         setTimeout(() => {
    62.           onREJECTED(this.result)
    63.         })
    64.       }
    65.     })
    66.   }
    67. }
    68. console.log("1");
    69. var p = new myPromise((resolve, reject) => {
    70.   console.log("2");
    71.   setTimeout(() => {
    72.     // console.log(p.status);
    73.     resolve("test")
    74.     reject("test-reject")
    75.     // console.log(p.status);
    76.     console.log("4");
    77.   })
    78.   // throw new Error("出错了")
    79. })
    80. p.then(
    81.   res => {
    82.     console.log("then-status", p.status);
    83.     console.log(res);
    84.   },
    85.   res => { console.log(res.message); }
    86. ).then(
    87.   res => { console.log("2-then", res); },
    88.   res => { console.log(res.message); }
    89. )
    90. console.log("3");