1> typeof
typeof是判断一个变量是否为str, Number, Boolean, undefined的最好方式
let str = 'str'
let num = 10
let flag = true
let u
let n = null
console.log( typeof str) // string
console.log( typeof num) // Number
console.log( typeof flag) // Boolean
console.log( typeof u) // undefined
console.log( typeof n) // Object
2> instanceof
instanceof判断是一个什么类型的对象
let arr = [1, 2, 3, 4]
let obj = {text:'文字'}
console.log(arr instanceof Array) // true
console.log(obj instanceof Object) // true
