typeof 运算符

识别所有值类型

识别函数

判断是否是引用类型（不可再细分）

let a;
const str = 'abc';
const n = 100;
const b = true;
const s = Symbol('s');

console.log(typeof a)
console.log(typeof str)
console.log(typeof n) 
console.log(typeof b) 
console.log(typeof s) 

//undefined
//string
//number
//boolean
//symbol

console.log(typeof console.log)
console.log(typeof function(){})

//function
//function

console.log(typeof null)
console.log(typeof ['a','b'])
console.log(typeof {x:100})

//object
//object
//object