作者：奥兰度

    1. // 异步 Promise + 错误处理 + 链式
    5. // new Promise(() => {}).then(() => { return 0}).then((val) => { /* 0 */ })
    7. class MyPromise {
    8.   constructor (executor) {
    9.     this.status = 'pending'
    10.     this.value = undefined // resolve value
    11.     this.reason = undefined // reject error
    12.     // 存放 then 成功和失败的回调，延后运行
    13.     this.onResolveCallbacks = [] // then(() => {}, () => {})
    14.     this.onRejectedCallbacks = [] // catch((err) => {})
    16.     let resolve = (value) => {
    17.       if (this.status === 'pending') {
    18.         this.status = 'resolved'
    19.         if (value instanceof MyPromise) {
    20.           setTimeout(()=> {
    21.             const then = value.then
    22.             if (typeof then === 'function') {
    23.               then.call(value, val => {
    24.                 this.value = val
    25.               }, err => {
    26.                 if (called) return
    27.                 called = true
    28.                 this.reason = err
    29.               })
    30.             }
    31.           })
    32.         } else {
    33.           this.value = value
    34.         }
    35.         // 收集的回调运行一下
    36.         this.onResolveCallbacks.forEach(fn => fn())
    37.       }
    38.     }
    39.     let reject = (reason) => {
    40.       if (this.status === 'pending') {
    41.         this.status = 'rejected'
    42.         this.reason = reason
    43.         // 收集的回调运行一下
    44.         this.onRejectedCallbacks.forEach(fn => fn())
    45.       }
    46.     }
    47.     try {
    48.       executor(resolve, reject)
    49.     } catch (e) {
    50.       reject(e)
    51.     }
    52.   }
    53.   then(onFullFilled, onRejected) {
    54.     onFullFilled = typeof onFullFilled === 'function' ? onFullFilled : value => value
    55.     onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }
    56.     let transPromise
    57.     transPromise = new MyPromise((resolve, reject) => {
    58.       if (this.status === 'resolved') {
    59.         try {
    60.           setTimeout(() => {
    61.             // resolve 后的 then 中 onFullFilled 返回的值是下一个 then 的 onFullFilled 接收的值
    62.             if (this.value instanceof MyPromise) {
    63.               this.value = this.value.value
    64.             }
    65.             const value = onFullFilled(this.value)
    66.             resolvePromise(transPromise, value, resolve, reject)
    67.           })
    68.         } catch (e) {
    69.           reject(e)
    70.         }
    71.       }
    72.       if (this.status === 'rejected') {
    73.         setTimeout(() => {
    74.           try {
    75.             const value = onRejected(this.reason)
    76.             resolvePromise(value)
    77.           } catch(e) {
    78.             reject(e)
    79.           }
    80.         })
    81.       }
    82.       // 收集回调，因为使用了 this 传递，所以值一直存在的
    83.       if (this.status === 'pending') {
    84.         this.onResolveCallbacks.push(() => {
    85.           setTimeout(() => {
    86.             try {
    87.               if (this.value instanceof MyPromise) {
    88.                 this.value = this.value.value
    89.               }
    90.               const value = onFullFilled(this.value)
    91.               resolvePromise(transPromise, value, resolve, reject)
    92.             } catch (e) {
    93.               reject(e)
    94.             }
    95.           })
    96.         })
    97.         this.onRejectedCallbacks.push(() => {
    98.           setTimeout(() => {
    99.             try {
    100.               const value = onRejected(this.reason)
    101.               resolvePromise(value)
    102.             } catch (e) {
    103.               reject(e)
    104.             }
    105.           })
    106.         })
    107.       }
    108.     })
    109.     function resolvePromise (transPromise, value, resolve, reject) {
    110.       if (transPromise === value) {
    111.         return reject(new TypeError('循环引用'))
    112.       }
    113.       let called // 是否调用成功或失败
    114.       // 看 value 是否是个 Promise
    115.       if (value !== null && (typeof value === 'object' || typeof value === 'function')) { // value instanceof MyPromise
    116.         try {
    117.           const then = value.then
    118.           if (typeof then === 'function') {
    119.             then.call(value, val => {
    120.               if (called) return
    121.               called = true
    122.               resolvePromise(transPromise, val, resolve, reject)
    123.             }, err => {
    124.               if (called) return
    125.               called = true
    126.               reject(err)
    127.             })
    128.           }
    129.         } catch (e) {
    130.           if (called) return
    131.           called = true
    132.           reject(e)
    133.         }
    134.       } else {
    135.         resolve(value)
    136.       }
    137.     }
    138.     return transPromise
    139.   }
    140.   defer () {
    141.     let dfd = {}
    142.     dfd.promise = new Promise((resolve, reject) => {
    143.       dfd.resolve = resolve
    144.       dfd.reject = reject
    145.     })
    146.     return dfd
    147.   }
    148.   deferred () {
    149.     this.defer()
    150.   }
    151. }
    155. const myPromiseT = new MyPromise((resolve, reject) => {
    156.     // setTimeout(() => {
    157.     resolve('foo');
    158.     // }, 300);
    159. });
    161. let promiseA = myPromiseT.then(res => {
    162.     console.log(res); return res
    163. })
    164. let promiseB = promiseA.then(res => {
    165.     console.log(res); return res
    166. })
    167. let promiseC = promiseB.then(res => {
    168.     console.log(res); return res
    169. })
    171. console.log('bar')

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