1. const PENDING = 'PEINDING'
    2. const REJECTED = 'REJECTED'
    3. const RESOLVE = 'RESOLVE'
    4. const reslovePromise = (promise2, x, resolve, reject) => {
    5.     let then = x.then;
    6.   let called;
    7.   if (typeof then === 'function') {
    8.       then.call(x, y => { // 根据 promise 的状态决定是成功还是失败
    9.       if (called) return;
    10.       called = true;
    11.       resolvePromise(promise2, y, resolve, reject); 
    12.     }, r => {
    13.       if (called) return;
    14.       called = true;
    15.       reject(r);
    16.     });
    17.   } else {
    18.        resolve(x);
    19.   }
    20. }
    22. Promise {
    23.     constructor (executor) {
    24.       this.status = PENGDING
    25.     this.value = null
    26.     this.reson = null
    27.     this.onRejectCallbacks = []
    28.     this.onResovlveCallBacks = []
    29.     let resolve = (value) {
    30.       if (this.staus === PENDING) {
    31.           this.status = RESOLVE
    32.         this.value = value
    33.         this.onResovleCallbacks.forEach((fn) => fn())
    34.       }
    35.     }
    36.     let reject = (reason) {
    37.         if (this.status === PENDING) {
    38.           this.staus = REJECTED
    39.         this.reason = reason
    40.         this.onRejectCallbacks.forEach((fn) => fn())
    41.       }
    42.     }
    43.     try {
    44.         executor(resolve, reject)
    45.     }catch (err){
    46.         reject(err)
    47.     }
    48.   }    
    49.   then (onResolve, onReject) {
    50.       onResolve = typeOf onResolve === 'function' ? onResolve : v => v
    51.     onReject = typeOf onReject === 'function' ? onReject : err => {throw err}
    52.     // then穿透
    54.     const promise2 = new Promise(async (resolve, reject) => {
    55.       if (this.staus === RESOLVE) {
    56.         try {
    57.             let x = await onResolve(this.value)
    58.             reslovePromise(promise2,x,resolve, reject)
    59.         } catch(err) {
    60.             reject(err)
    61.         }
    62.       }
    63.       if (this.status === REJECTED) {
    64.         try {
    65.             let x = await onResolve(this.reason)
    66.             reslovePromise(promise2,x,resolve, reject)
    67.         } catch(err) {
    68.             reject(err)
    69.         }
    70.       }
    71.       if (this.status === PENDING) {
    72.         this.onResolveCallbacks.push(() => {
    73.           try {
    74.             let x = await onResolve(this.reason)
    75.             reslovePromise(promise2,x,resolve, reject)
    76.           } catch(err) {
    77.             reject(err)
    78.           }
    79.         })
    80.         this.onRejectCallbacks.push(() => {
    81.           try {
    82.             let x = await onResolve(this.reason)
    83.             reslovePromise(promise2,x,resolve, reject)
    84.           } catch(err) {
    85.             reject(err)
    86.           }
    87.         })
    88.       }
    89.     })
    90.     return promise2
    91.   }
    92. }