手撕一个营养不良的Promise

function Promise(executor){ // 传进来的是个函数哦
	let _this = this;
	this.state = PENDING; // 初始状态
	this.value = undefined; // 初始化 //成功结果
	this.reason = undefined; // 初始化 失败原因
	this.onFulfilled = []; // 成功的回调
	this.onRejected = []; // 失败的回调
	function resolve(value){ 
		if(_this.state===PENDING){
			_this.state = FULFILLED
			_this.value = value
			_this.onFulfilled.forEach(f=>f(value))
		}
	}

	function reject(reason){
		if(_this.state === PENDING){
			_this.state = REJETED
			_this.reason = reason
			_this.onRejected.forEach(f=>f(reason))
		}
	}
	try{ // 为了捕获构造异常
		executor(resolve,reject) 
	}catch (e){
		reject(e)
	}
	Promise.prototype.then = function (onFulfilled,onRejected){ // 定义链式调用then方法
		if(_this.state === FULFILLED){
			typeof onFulfilled === 'function' && onFulfilled(_this.value)
		}
		if(_this.state === REJETED){
			typeof onRejected === 'function' && onRejected(_this.reason)
		}
		if(_this.state === PENDING){
			typeof onFulfilled === 'function' && _this.onFulfilled.push(onFulfilled)
			typeof onRejected === 'function' && _this.onRejected.push(onRejected)
		}
	}
 }

　　基本上面试的话，有以上描述就差不多了