手写Promise

首先，要手写Promise，那必须先了解它的功能和自己打算用的方法。这里推荐先阅读阮一峰的《Promise 对象》以及《Class 的基本语法》

先上代码（待补充异步），这部分已实现resolve、reject、then、catch等功能

    class MyPromise {
        static PENDING = 'pending'
        static FULFILLED = 'fulfilled'
        static REJECTED = 'rejected'

        constructor(func) {
            this.PromiseState = MyPromise.PENDING
            this.PromiseResult = null
            try {
                func(this.resolve.bind(this), this.reject.bind(this))
            } catch (error) {
                this.reject(error)
            }
        }

        resolve(result) {
            if (this.PromiseState === MyPromise.PENDING) {
                this.PromiseState = MyPromise.FULFILLED
                this.PromiseResult = result
            }
        }

        reject(reason) {
            if (this.PromiseState === MyPromise.PENDING) {
                this.PromiseState = MyPromise.REJECTED
                this.PromiseResult = reason
            }
        }

        then(onFulfilled, onRejected) { 
            onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value
            onRejected = typeof onRejected === 'function' ? onRejected : reason => {
                throw reason
            }
            if (this.PromiseState === MyPromise.FULFILLED) {
                onFulfilled(this.PromiseResult)
            }

            if (this.PromiseState === MyPromise.REJECTED) {
                onRejected(this.PromiseResult)
            }
        }
    }

    let promise1 = new MyPromise((resolve, reject) => {
        resolve('success')
    })
    promise1.then(result => {
        console.log('result:', result)
    }, reason => {
        console.log('reason:', reason )
    })
    console.log(promise1)

 

Promise 对象