js判断一个元素是否在数组中

1.比较传统的做法：

概述:  indexOf()方法返回给定元素能找在数组中找到的第一个索引值，否则返回-1。

var a=["1", "3", "2", "3", "0"];
      console.log(a.indexOf (\'3\'));//1
      console.log(a.indexOf (5,\'0\'));//-1
      console.log(a.indexOf (\'0\'));//4
      console.log(a.indexOf (\'0\',-5));//4
      console.log(a.indexOf (\'3\',-1));//-1
      console.log(a.indexOf (\'3\',-3));//3
      console.log(a.indexOf (\'1\',5));//-1
      console.log(a.indexOf (3));//-1
      console.log(a);//["1", "3", "2", "3", "0"]

2.今天发现有个函数比较好用

let arr = [1,2,3];
arr.includes(1) // true
arr.includes(5) //false