百度面经（1）

class myPromise {
    state = 'pending'
    value = undefined
    reason = undefined
    onFulfilledCallback = []
    onRejectedCallback = []
    constructor(executor) {
        this.resolve = this.resolve.bind(this)
        this.reject = this.reject.bind(this)
        try{
            executor(this.resolve, this.reject)
        }catch(err){
            this.reject(err)
        }
    }
    resolve(value){
        if (this.state === 'pending') {
            this.state = 'fulfilled'
            this.value = value
            while(this.onFulfilledCallback.length){
                this.onFulfilledCallback.shift()(value)
            }
        }
    }
    reject(reason){
        if (this.state === 'pending') {
            this.state = 'rejected'
            this.reason = reason
            while(this.onRejectedCallback.length){
                this.onRejectedCallback.shift()(reason)
            }
        }
    }
    then(onFulfilled, onRefected) {
        if (this.state === 'fulfilled') {
            onFulfilled(this.value)
        }
        if (this.state === 'rejected') {
            onRefected(this.reason)
        }
        if(this.state === 'pending'){
            this.onFulfilledCallback.push(onFulfilled)
            this.onRejectedCallback.push(onRefected)
        }   
    }
}