1.Promise有三个状态 pending待决议 fulfilled成功 rejected失败，一旦改变不能再修改

2.Promise构造函数接受一个函数，参数分别未resolve、reject两个函数，用
于修改状态

3.有一个then方法，接受两个回调,状态变化时调用相应的函数，并返回新的
promise对象

4.有一个catch方法，接收一个失败回调

const PENDING="pending"
const FULFILLED="fulfilled"
const REJECTED="rejected"

function _isFunction(fn){
    return typeof fn ==='function'
}
class Promise{
    constructor(fn) {
        this.state=PENDING
        this.success=undefined
        this.fail=undefined
        this.resolve_arg=undefined
        this.reject_arg=undefined
        this.nextState=PENDING
        this.next_resolve_arg=undefined
        this.next_reject_arg=undefined
        this.nextResolve=undefined
        this.nextReject=undefined
        fn(this.resolve.bind(this),this.reject.bind(this))
    }
    resolve(arg){
        this.resolve_arg=arg
        this.state=FULFILLED
        if (this.success){
            this.next_resolve_arg=this.success(arg)
            if (this.nextResolve){
                this.nextResolve()
            }
        }

    }
    reject(arg){
        this.reject_arg=arg
        this.state=REJECTED
        if (this.fail){
            this.next_reject_arg=this.fail(arg)
        }else {
            if (this.nextReject){
                this.nextReject(this.next_reject_arg)
            }
        }

    }

    then(success,fail){
        if (this.state===FULFILLED){
            this.next_resolve_arg=success(this.resolve_arg)
            this.nextState=FULFILLED
        }else if (this.state===REJECTED){
            this.next_reject_arg=fail(this.reject_arg)
            this.nextState=REJECTED
        }else {
            this.success=success
            this.fail=fail
        }

        let $this=this
        return new Promise(function (n_resolve,n_reject){
            if ($this.nextState===FULFILLED){
                n_resolve($this.next_resolve_arg)
            }else if ($this.nextState===REJECTED){
                n_reject($this.next_reject_arg)
            }else {
                $this.nextResolve=n_resolve
                $this.nextReject=n_reject
            }
        })
    }

    catch(fail){
        if (this.state===REJECTED){
            fail(this.reject_arg)
        }else {
            this.fail=fail
        }
    }
}
module.exports=Promise