JS中类型判断其实是个很麻烦的事情，特别是在各种情况下都能准确的获得变量的类型是挺不容易的，以下介绍一种比较好记忆的办法

• 原始类型用 typeof

var a = undefined;
var b = null;
var c = true;
var d = 1;
var e = 'string';
var f = function(){}
console.log(typeof a);  //undefined
console.log(typeof b);  //object
console.log(typeof c);  //boolean
console.log(typeof d);  //number
console.log(typeof e);  //string
console.log(typeof f);   //function

• 引用类型用 instanceof

var f = [];
var g = function(){};
var h = {};
console.log(f instanceof Array);       //true
console.log(g instanceof Function);    //true
console.log(h instanceof Object)       //true

• 万能方法用 String.prototype.toString.call

var a = undefined;
var b = null;
var c = true;
var d = 1;
var e = 'string';
var f = [];
var g = function(){};
var h = {};
console.log(Object.prototype.toString.call(a))    //[object Undefined]
console.log(Object.prototype.toString.call(b))    //[object Null]
console.log(Object.prototype.toString.call(c))    //[object Boolean]
console.log(Object.prototype.toString.call(d))    //[object Number]
console.log(Object.prototype.toString.call(e))    //[object String]
console.log(Object.prototype.toString.call(f))    //[object Array]
console.log(Object.prototype.toString.call(g))    //[object Function]
console.log(Object.prototype.toString.call(h))    //[object Object]

类库中一般是使用第三种办法判断