Promise方法

class MyPomise {
constructor(handle){
    if(typeof handle!=='function){
        throw new Error('MyPromise must accept a function as a parameter')
    }
    //添加状态
    this._status='PENDING'
    //添加值
    this._val=undefined

    // 执行handle
    try{
        handle(this._resolve.bind(this),this._reject.bind(this))
    }catch(err){
        this.reject(err)
    }
}
    _resolve(val){
        if(this._status!=='PENDING'){
            return
        }
        this._status='FULFILLED'
        this._val=val
    }
    _reject(err){
        if(this._status!=='PENDING'){
            return
        }
        this._status='REJECTED'
        this._val=err
    }
}