AHABHGK

1const isFunction = obj => typeof obj === 'function'
2const isObject = obj => !!(obj && typeof obj === 'object') // null 的情况
3const isThenable = obj => (isFunction(obj) || isObject(obj)) && 'then' in obj && isFunction(obj.then)
4const isPromise = promise => promise instanceof Promise
5
6const PENDING = 'pending'
7const FULFILLED = 'fulfilled'
8const REJECTED = 'rejected'
9
10class Promise {
11  constructor(fn) {
12    this.status = PENDING
13    this.value = undefined
14    this.reason = undefined
15    this.onFulfilledCallbacks = []
16    this.onRejectedCallbacks = []
17    function resolve(value) {
18      if (this.status !== PENDING)
19        return
20      setTimeout(() => {
21        this.status = FULFILLED
22        this.value = value
23        this.onFulfilledCallbacks.forEach(cb => cb(this.value))
24      }, 0)
25    }
26    function reject(reason) {
27      if (this.status !== PENDING)
28        return
29      setTimeout(() => {
30        this.status = REJECTED
31        this.reason = reason
32        this.onRejectedCallbacks.forEach(cb => cb(this.reason))
33      })
34    }
35    try {
36      fn(resolve, reject)
37    } catch (e) {
38      reject(e)
39    }
40  }
41
42  then(onFulfilled, onRejected) {
43    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value
44    onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason }
45    return bridgePromise = new Promise((resolve, reject) => {
46      if (this.status === FULFILLED) {
47        setTimeout(() => {
48          try {
49            let result = onFulfilled(this.value)
50            resolvePromise(bridgePromise, result, resolve, reject)
51          } catch (e) {
52            reject(e)
53          }
54        }, 0)
55      } else if (this.status === REJECTED) {
56        setTimeout(() => {
57          try {
58            let result = onRejected(this.reason)
59            resolvePromise(bridgePromise, result, resolve, reject)
60          } catch (e) {
61            reject(e)
62          }
63        }, 0)
64      } else if (this.status === PENDING) {
65        this.onFulfilledCallbacks.push(() => {
66          try {
67            let result = onFulfilled(this.value)
68            resolvePromise(bridgePromise, result, resolve, reject)
69          } catch (e) {
70            reject(e)
71          }
72        })
73        this.onRejectedCallbacks.push(() => {
74          try {
75            let result = onRejected(this.reason)
76            resolvePromise(bridgePromise, result, resolve, reject)
77          } catch (e) {
78            reject(e)
79          }
80        })
81      }
82    })
83  }
84
85  catch(onRejected) {
86    return this.then(null, onRejected)
87  }
88
89  static resolve(p) {
90    if (isPromise(p)) return p
91    return new Promise((resolve, reject) => {
92      if (isThenable(p)) p.then(resolve, reject)
93      else resolve(p)
94    })
95  }
96
97  static reject(p) {
98    return new Promise((_, reject) => reject(p))
99  }
100
101  static all(promises) {
102    return new Promise((resolve, reject) => {
103      let values = []
104      let count = 0
105      function handle(value, index) {
106        values[index] = value
107        if (++count === promises.length) resolve(values)
108      }
109      // p 可能不是 Promise，所以用 Promise.resolve 包一下
110      promises.forEach((p, i) => Promise.resolve(p).then(value => handle(value, i), reject))
111    })
112  }
113
114  static race(promises) {
115    return new Promise((resolve, reject) => {
116      promises.forEach(p => Promise.resolve(p).then(resolve, reject))
117    })
118  }
119
120  static allSettled(promises) {
121    return new Promise((resolve) => {
122      let results = []
123      let count = 0
124      function handle(result, index) {
125        results[index] = result
126        if (++count === promises.length) resolve(results)
127      }
128      promises.forEach((p, i) => Promise.resolve(p).then(
129        value => handle({ status: 'fulfilled', value }, i),
130        reason => handle({ status: 'rejected', reason }, i),
131      ))
132    })
133  }
134}
135
136function resolvePromise(bridgePromise, result, resolve, reject) {
137  if (bridgePromise === result) {
138    return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
139  }
140  if (isPromise(result)) {
141    if (result.status === PENDING) {
142      result.then(y => resolvePromise(bridgePromise, y, resolve, reject), reject)
143    } else {
144      result.then(resolve, reject)
145    }
146  } else if (isThenable(result)) {
147    result.then(y => resolvePromise(bridgePromise, y, resolve, reject), reject)
148  } else {
149    resolve(result)
150  }
151}