typeof()用于判断数据类型，数据类型有: ①基本数据类型:string number null undefined boolean ②复杂数据类型:object typeof只能检测数据类型，而不能检测从数据类型中派生的其他类型，数组为object派生对象，所以依然返回object function fn(){
console.log(1);
}
console.log(typeof []);//object
console.log(typeof '[]');//string
console.log(typeof 1);//number
console.log(typeof true);//boolean
console.log(typeof null);//object
console.log(typeof undefined);//undefined
console.log(typeof {id:2});//object
console.log(typeof fn);//function
