js中调用this，为什么总打印一个undefined呢？

	var obj={
		fun1:function(){
			console.log(this)
		},
		fun2(){
			console.log(this)
		},
		fun3:()=>{
			console.log(this)
		}
	}

	console.log(obj.fun1())
	console.log(obj.fun2())
	console.log(obj.fun3())