Promise Polyfill (Promises/A+ Spec)

1const PENDING = 'PENDING';
2const FULFILLED = 'FULFILLED';
3const REJECTED = 'REJECTED';
4
5class MyPromise {
6  constructor(executor) {
7    this.state = PENDING;
8    this.value = undefined;
9    this.handlers = [];
10
11    const resolve = (val) => {
12      if (this.state !== PENDING) return;
13      if (val && typeof val.then === 'function') {
14        return val.then(resolve, reject);
15      }
16      this.state = FULFILLED;
17      this.value = val;
18      this.executeHandlers();
19    };
20
21    const reject = (reason) => {
22      if (this.state !== PENDING) return;
23      this.state = REJECTED;
24      this.value = reason;
25      this.executeHandlers();
26    };
27
28    try {
29      executor(resolve, reject);
30    } catch (err) {
31      reject(err);
32    }
33  }
34
35  executeHandlers() {
36    if (this.state === PENDING) return;
37    queueMicrotask(() => {
38      for (const { onFulfilled, onRejected, resolveNext, rejectNext } of this.handlers) {
39        try {
40          if (this.state === FULFILLED) {
41            if (typeof onFulfilled === 'function') {
42              resolveNext(onFulfilled(this.value));
43            } else {
44              resolveNext(this.value);
45            }
46          } else if (this.state === REJECTED) {
47            if (typeof onRejected === 'function') {
48              resolveNext(onRejected(this.value));
49            } else {
50              rejectNext(this.value);
51            }
52          }
53        } catch (err) {
54          rejectNext(err);
55        }
56      }
57      this.handlers = [];
58    });
59  }
60
61  then(onFulfilled, onRejected) {
62    return new MyPromise((resolveNext, rejectNext) => {
63      this.handlers.push({ onFulfilled, onRejected, resolveNext, rejectNext });
64      this.executeHandlers();
65    });
66  }
67
68  catch(onRejected) {
69    return this.then(null, onRejected);
70  }
71}
72
73const logs = [];
74logs.push('sync-start');
75
76new MyPromise((res, rej) => {
77  res(10);
78  rej('ignored');
79})
80  .then((val) => {
81    logs.push('val:' + val);
82    return val * 2;
83  })
84  .then((val) => {
85    logs.push('val:' + val);
86    throw new Error('boom');
87  })
88  .catch((err) => {
89    logs.push('caught:' + err.message);
90    return 'recovered';
91  })
92  .then((val) => {
93    logs.push('final:' + val);
94  });
95
96logs.push('sync-end');
97
98queueMicrotask(() => {
99  queueMicrotask(() => {
100    queueMicrotask(() => {
101      queueMicrotask(() => {
102        console.log(logs.join(' '));
103      });
104    });
105  });
106});