typeof
var a = 100; console.log(typeof a); //输出 number
var a = 'hello'; console.log(typeof a); //输出 string
var a = true; console.log(typeof a); //输出 boolean
var a = undefined; console.log(typeof a); //输出 undefined
var a = null; console.log(typeof a); //输出 object
var a = new Object(); console.log(typeof a); //输出 object
var a = Object; console.log(typeof a) //输出 function