- includes:判断某个值是否在数组中,返回true/false
- indexOf():返回该值在数组中的索引,如果没有则返回-1
- in:判断索引是否在数组中,比如0,1,2,3。注意不是值,而是索引
let arr = ['apple','banana','pear']
console.log('apple' in arr) //false
console.log(1 in arr) // true
console.log(arr.indexOf("apple")) // 0
console.log(arr.includes('pear')) // true
console.log(arr.includes(1)) // false