简易实现

    1. function Promise(executor) {
    2.   this.status = 'pending'
    3.   this.value = null
    4.   this.reason = null
    5.   this.onFulfilledArray = []
    6.   this.onRejectedArray = []
    8.   const resolve = value => {
    9.     if (value instanceof Promise) {
    10.       return value.then(resolve, reject)
    11.     }
    12.     setTimeout(() => {
    13.       if (this.status === 'pending') {
    14.         this.value = value
    15.         this.status = 'fulfilled'
    17.         this.onFulfilledArray.forEach(func => {
    18.           func(value)
    19.         })
    20.       }
    21.     })
    22.   }
    24.   const reject = reason => {
    25.     setTimeout(() => {
    26.       if (this.status === 'pending') {
    27.         this.reason = reason
    28.         this.status = 'rejected'
    30.         this.onRejectedArray.forEach(func => {
    31.           func(reason)
    32.         })
    33.       }
    34.     })
    35.   }
    38.   try {
    39.     executor(resolve, reject)
    40.   } catch(e) {
    41.     reject(e)
    42.   }
    43. }
    45. Promise.prototype.then = function(onfulfilled, onrejected) {
    46.   onfulfilled = typeof onfulfilled === 'function' ? onfulfilled : data => data
    47.   onrejected = typeof onrejected === 'function' ? onrejected : error => { throw error}
    49.   if (this.status === 'fulfilled') {
    50.     onfulfilled(this.value)
    51.   }
    52.   if (this.status === 'rejected') {
    53.     onrejected(this.reason)
    54.   }
    55.   if (this.status === 'pending') {
    56.     this.onFulfilledArray.push(onfulfilled)
    57.     this.onRejectedArray.push(onrejected)
    58.   }
    59. }

    详细实现

    1. function Promise(executor) {
    2.   this.status = 'pending'
    3.   this.value = null
    4.   this.reason = null
    5.   this.onFulfilledArray = []
    6.   this.onRejectedArray = []
    8.   const resolve = value => {
    9.     if (value instanceof Promise) {
    10.       return value.then(resolve, reject)
    11.     }
    12.     setTimeout(() => {
    13.       if (this.status === 'pending') {
    14.         this.value = value
    15.         this.status = 'fulfilled'
    17.         this.onFulfilledArray.forEach(func => {
    18.           func(value)
    19.         })
    20.       }
    21.     })
    22.   }
    24.   const reject = reason => {
    25.     setTimeout(() => {
    26.       if (this.status === 'pending') {
    27.         this.reason = reason
    28.         this.status = 'rejected'
    30.         this.onRejectedArray.forEach(func => {
    31.           func(reason)
    32.         })
    33.       }
    34.     })
    35.   }
    38.     try {
    39.         executor(resolve, reject)
    40.     } catch(e) {
    41.         reject(e)
    42.     }
    43. }
    45. const resolvePromise = (promise2, result, resolve, reject) => {
    46.   // 当 result 和 promise2 相等时，也就是说 onfulfilled 返回 promise2 时，进行 reject
    47.   if (result === promise2) {
    48.     return reject(new TypeError('error due to circular reference'))
    49.   }
    51.   // 是否已经执行过 onfulfilled 或者 onrejected
    52.   let consumed = false
    53.   let thenable
    55.   if (result instanceof Promise) {
    56.     if (result.status === 'pending') {
    57.       result.then(function(data) {
    58.         resolvePromise(promise2, data, resolve, reject)
    59.       }, reject)
    60.     } else {
    61.       result.then(resolve, reject)
    62.     }
    63.     return
    64.   }
    66.   let isComplexResult = target => (typeof target === 'function' || typeof target === 'object') && (target !== null)
    67.   // 如果返回的是疑似 Promise 类型
    68.   if (isComplexResult(result)) {
    69.     try {
    70.       thenable = result.then
    71.       // 如果返回的是 Promise 类型，具有 then 方法
    72.       if (typeof thenable === 'function') {
    73.         thenable.call(result, function(data) {
    74.           if (consumed) {
    75.             return
    76.           }
    77.           consumed = true
    79.           return resolvePromise(promise2, data, resolve, reject)
    80.         }, function(error) {
    81.           if (consumed) {
    82.             return
    83.           }
    84.           consumed = true
    86.           return reject(error)
    87.         })
    88.       }
    89.       else {
    90.         return resolve(result)
    91.       }
    93.     } catch(e) {
    94.       if (consumed) {
    95.         return
    96.       }
    97.       consumed = true
    98.       return reject(e)
    99.     }
    100.   }
    101.   else {
    102.     return resolve(result)
    103.   }
    104. }
    106. Promise.prototype.then = function(onfulfilled, onrejected) {
    107.   onfulfilled = typeof onfulfilled === 'function' ? onfulfilled : data => data
    108.   onrejected = typeof onrejected === 'function' ? onrejected : error => {throw error}
    110.   // promise2 将作为 then 方法的返回值
    111.   let promise2
    113.   if (this.status === 'fulfilled') {
    114.     return promise2 = new Promise((resolve, reject) => {
    115.       setTimeout(() => {
    116.         try {
    117.           // 这个新的 promise2 resolved 的值为 onfulfilled 的执行结果
    118.           let result = onfulfilled(this.value)
    119.           resolvePromise(promise2, result, resolve, reject)
    120.         }
    121.         catch(e) {
    122.           reject(e)
    123.         }
    124.       })
    125.     })
    126.   }
    127.   if (this.status === 'rejected') {
    128.     return promise2 = new Promise((resolve, reject) => {
    129.       setTimeout(() => {
    130.         try {
    131.           // 这个新的 promise2 reject 的值为 onrejected 的执行结果
    132.          let result = onrejected(this.reason)
    133.          resolvePromise(promise2, result, resolve, reject)
    134.         }
    135.         catch(e) {
    136.           reject(e)
    137.         }
    138.       })
    139.     })
    140.   }
    141.   if (this.status === 'pending') {
    142.     return promise2 = new Promise((resolve, reject) => {
    143.       this.onFulfilledArray.push(value => {
    144.         try {
    145.           let result = onfulfilled(value)
    146.           resolvePromise(promise2, result, resolve, reject)
    147.         }
    148.         catch(e) {
    149.           return reject(e)
    150.         }
    151.       })
    153.       this.onRejectedArray.push(reason => {
    154.         try {
    155.           let result = onrejected(reason)
    156.           resolvePromise(promise2, result, resolve, reject)
    157.         }
    158.         catch(e) {
    159.           return reject(e)
    160.         }
    161.       })      
    162.     })
    163.   }
    164. }
    166. Promise.prototype.catch = function(catchFunc) {
    167.   return this.then(null, catchFunc)
    168. }
    170. Promise.resolve = function(value) {
    171.   return new Promise((resolve, reject) => {
    172.     resolve(value)
    173.   })
    174. }
    176. Promise.reject = function(value) {
    177.   return new Promise((resolve, reject) => {
    178.     reject(value)
    179.   })
    180. }
    182. Promise.race = function(promiseArray) {
    183.   if (!Array.isArray(promiseArray)) {
    184.       throw new TypeError('The arguments should be an array!')
    185.   }
    186.   return new Promise((resolve, reject) => {
    187.     try {
    188.       const length = promiseArray.length
    189.       for (let i = 0; i <length; i++) {
    190.         promiseArray[i].then(resolve, reject)
    191.       }
    192.     }
    193.     catch(e) {
    194.       reject(e)
    195.     }
    196.   })
    197. }
    199. Promise.all = function(promiseArray) {
    200.   if (!Array.isArray(promiseArray)) {
    201.       throw new TypeError('The arguments should be an array!')
    202.   }
    203.   return new Promise((resolve, reject) => {
    204.     try {
    205.       let resultArray = []
    207.       const length = promiseArray.length
    209.       for (let i = 0; i <length; i++) {
    210.         promiseArray[i].then(data => {
    211.           resultArray.push(data)
    213.           if (resultArray.length === length) {
    214.             resolve(resultArray)
    215.           }
    216.         }, reject)
    217.       }
    218.     }
    219.     catch(e) {
    220.       reject(e)
    221.     }
    222.   })
    223. }