类型检测的方法
typeof
instanceof
Object.protype.toString
constructor
duck type:鸭子类型
typeof
返回一个字符串，适合函数对象和基本类型的判断
var a = 123;var b = '123';var c = false;var d = null;var e = undefined;var f = {};var g = function(){};var h = [];var i = NaN;console.log(typeof a);console.log(typeof b);console.log(typeof c);console.log(typeof d);console.log(typeof e);console.log(typeof f);console.log(typeof g);console.log(typeof h);console.log(typeof i); 
instanceof
返回一个布尔值，判断对象类型（基于原型链的方式）
原理：
左边对象的原型链（_proto_原型）上是否有右边构造函数的prototype对象属性
var a = 123; var b = '123'; var c = false; var d = null; var e = undefined; var f = {}; var g = function(){}; var h = []; var i = NaN; console.log(a instanceof Number); console.log(b instanceof String); console.log(c instanceof Boolean); console.log(d instanceof Object); console.log(e instanceof Object); console.log(f instanceof Object); console.log(g instanceof Function); console.log(h instanceof Array); console.log(i instanceof Number);//NaN是基本类型 这么多false就说明instanceof不适合用于基本数据类型！！
Object.prototype.toString
var oP = Object.prototype,toString = oP.toString;console.log(toString.call(123));//[object Number]console.log(toString.call('123'));//[object String]console.log(toString.call(false));//[object Boolean]console.log(toString.call(undefined));//[object Undefined]console.log(toString.call(null));//[object Null]console.log(toString.call({a: '123'}));//[object Object]console.log(toString.call(/123/));//[object RegExp]console.log(toString.call([123]));//[object Array]console.log(toString,call(Date));//[object Function] 注意：IE6/7/8 Object.prototype.toString.call(null/undefined)返回的是[object Object]
constructor
var a = 123;var b = '123';var c = false;var d = null;var e = undefined;var f = {};var g = function(){};var h = [];var i = NaN;console.log(a.constructor == Number);console.log(b.constructor == String);console.log(c.constructor == Boolean);// console.log(d.constructor == Object);// console.log(e.constructor == Object);console.log(f.constructor == Object);console.log(g.constructor == Function);console.log(h.constructor == Array);console.log(i.constructor == Number); 