Promise的模拟实现

```js const PENDING = 'pending' const RESOLVED = 'resolved' const REJECTED = 'rejected'

function MyPromise(fn){ const that = this that.state = PENDING that.value = null that.resolvedCallbacks = [] that.rejectedCallbacks = []

function resolve(value) { if(that.state === PENDING) { that.state = RESOLVED that.value = value that.resolvedCallbacks.map(cb => cb(that.value)) } }

function reject(value) { if(that.state === PENDING){ that.state = REJECTED that.value = value; that.rejectedCallbacks.map(cb => cb(that.value)); } } try { fn(resolve, reject) } catch (e) { reject(e) } }

MyPromise.prototype.then = function(onFulfilled, onRejected) { const that = this //对传入的两个参数做判断，如果不是函数将其转为函数 onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v // onFulfilled = v => v onRejected = typeof onRejected === 'function' ? onRejected : r => { throw r }

if(that.state === PENDING) { that.resolvedCallbacks.push(onFulfilled) that.rejectedCallbacks.push(onRejected) } else if(that.state === RESOLVED) { onFulfilled(that.value) } else { onRejected(that.value) } }

new MyPromise((resolve, reject) => { setTimeout(() => { resolve('成功的回调数据') }, 1000) }).then(value => { console.log('Promise.then: ', value) }) ```