js数据类型检测的四种方式

1.typeof

typeof 用来检测数据类型的运算符（返回一个字符串，适合函数对象和基本类型的判断）
   检测的不管是数组还是正则都返回的是"object",所以typeof不能判断一个值是否为数组

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);//number
console.log(typeof b);//string
console.log(typeof c);//boolean
console.log(typeof d);//object
console.log(typeof e);//undefined
console.log(typeof f);//object
console.log(typeof g);function
console.log(typeof h);//object
console.log(typeof i);//number
注：console.log(typeof typeof typeof [12, 23]);//string 两个或多个typeof一起使用时，返回string

2.instanceof

返回一个布尔值，判断对象类型（基于原型链的方式）
我们使用instanceof/constructor可以检测数组和正则了
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
不适用于基本数据类型。

    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);//false
    console.log(b instanceof String);//false
    console.log(c instanceof Boolean);//false
    console.log(d instanceof Object);//false
    console.log(e instanceof Object);//false
    console.log(f instanceof Object);//true
    console.log(g instanceof Function);true
    console.log(h instanceof Array);//true
    console.log(i instanceof Number);//false NaN是基本数据类型

注意：alert(typeof(null)); //object​ alert(null instanceof Object); //false

3.Object.prototype.toString.call()

在Object基本类定义的这个toString()方法，是用来检测数据类型的；
Object.prototype.toString是用来返回对应值的所属类信息的
跟字符串、数字、布尔等原型上定义的toString()方法基本用法都是转换字符串的。

console.log(Object.prototype.toString.call(1));//[object Number]
console.log(Object.prototype.toString.call(''));//[object String]
console.log(Object.prototype.toString.call(true));//[object Boolean]
console.log(Object.prototype.toString.call(null));// [object Null]
console.log(Object.prototype.toString.call(undefined));//[object Undefined]
console.log(Object.prototype.toString.call([]));// [object Array]
console.log(Object.prototype.toString.call({}));// [object Object]
console.log(Object.prototype.toString.call(/^$/));//[object RegExp]
console.log(Object.prototype.toString.call((function () {})));//[object Function]
注意：IE6/7/8 Object.prototype.toString.call(null/undefined)返回的是[object Object]

4.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);//true
console.log(b.constructor == String);//true
console.log(c.constructor == Boolean);//true
// console.log(d.constructor == Object);//true
// console.log(e.constructor == Object);//true
console.log(f.constructor == Object);//true
console.log(g.constructor == Function);//true
console.log(h.constructor == Array);//true
console.log(i.constructor == Number);//true