typeof和instanceof

typeof

基本数据类型：Undefined、Null、Boolean、Number 和 String

• Undefined、Boolean、Number 、String返回对应类型
• NaN返回number类型
• null返回 object
• new函数实例和对象返回object
• 函数和class构造函数返回function

console.log(typeof a);    //'undefined'
console.log(typeof(true));  //'boolean'
console.log(typeof '123');  //'string'
console.log(typeof 123);   //'number'
console.log(typeof NaN);   //'number'
console.log(typeof null);  //'object'    
var obj = new String();
console.log(typeof(obj));    //'object'
var  fn = function(){};
console.log(typeof(fn));  //'function'
console.log(typeof(class c{}));  //'function'

instanceof

• instanceof返回true/false
• instanceof代表原型，依靠对象proto追溯

console.log(a instanceof Object)
console.log(a instanceof Array)