javascript中4种类型识别的方法

typeof

typeof操作符用于返回正在使用值的类型

【输出】首字母小写的字符串形式

【功能】

[1]可以识别标准类型(将Null识别为object)

[2]不能识别具体的对象类型(Function除外)

【实例】

console.log(typeof "jerry");//"string"  console.log(typeof 12);//"number"  console.log(typeof true);//"boolean"  console.log(typeof undefined);//"undefined"  console.log(typeof null);//"object"  console.log(typeof {name: "jerry"});//"object"    console.log(typeof function(){});//"function"  console.log(typeof []);//"object"  console.log(typeof new Date);//"object"  console.log(typeof /\d/);//"object"  function Person(){};  console.log(typeof new Person);//"object"

Object.prototype.toString

返回代表该对象的字符串表示

【输出】[object 数据类型]的字符串形式

【功能】

[1]可以识别标准类型及内置对象类型

[2]不能识别自定义类型

【构造方法】

function type(obj){  　　return Object.prototype.toString.call(obj).slice(8,-1).toLowerCase();  }    

【实例1】

console.log(Object.prototype.toString.call("jerry"));//[object String]  console.log(Object.prototype.toString.call(12));//[object Number]  console.log(Object.prototype.toString.call(true));//[object Boolean]  console.log(Object.prototype.toString.call(undefined));//[object Undefined]  console.log(Object.prototype.toString.call(null));//[object Null]  console.log(Object.prototype.toString.call({name: "jerry"}));//[object Object]    console.log(Object.prototype.toString.call(function(){}));//[object Function]  console.log(Object.prototype.toString.call([]));//[object Array]  console.log(Object.prototype.toString.call(new Date));//[object Date]  console.log(Object.prototype.toString.call(/\d/));//[object RegExp]  function Person(){};  console.log(Object.prototype.toString.call(new Person));//[object Object]

【实例2】

function type(obj){      return Object.prototype.toString.call(obj).slice(8,-1).toLowerCase();  }  console.log(type("jerry"));//"string"  console.log(type(12));//"number"  console.log(type(true));//"boolean"  console.log(type(undefined));//"undefined"  console.log(type(null));//"null"  console.log(type({name: "jerry"}));//"object"    console.log(type(function(){}));//"function"  console.log(type([]));//"array"  console.log(type(new Date));//"date"  console.log(type(/\d/));//"regexp"  function Person(){};  console.log(type(new Person));//"object"

constructor

构造函数实例拥有指向其构造函数的constructor属性

【输出】function 数据类型(){[native code]}或者function 自定义类型(){}

【功能】

[1]可以识别标准类型、内置对象类型及自定义类型

[2]不能识别undefined、null，会报错

【构造方法】

function type(obj){      var temp = obj.constructor.toString();      return temp.replace(/^function (\w+)\(\).+$/,'$1');  }

【实例1】

console.log(("jerry").constructor);//function String(){[native code]}  console.log((12).constructor);//function Number(){[native code]}  console.log((true).constructor);//function Boolean(){[native code]}  console.log((undefined).constructor);//报错  console.log((null).constructor);//报错  console.log(({name: "jerry"}).constructor);//function Object(){[native code]}    console.log((function(){}).constructor);//function Function(){[native code]}  console.log(([]).constructor);//function Array(){[native code]}  console.log((new Date).constructor);//function Date(){[native code]}  console.log((/\d/).constructor);//function RegExp(){[native code]}  function Person(){};  console.log((new Person).constructor);//function Person(){}

【实例2】

function type(obj){      var temp = obj.constructor.toString().toLowerCase();      return temp.replace(/^function (\w+)\(\).+$/,'$1');  }  console.log(type("jerry"));//"string"  console.log(type(12));//"number"  console.log(type(true));//"boolean"  console.log(type(undefined));//错误  console.log(type(null));//错误  console.log(type({name: "jerry"}));//"object"    console.log(type(function(){}));//"function"  console.log(type([]));//"array"  console.log(type(new Date));//"date"  console.log(type(/\d/));//"regexp"  function Person(){};  console.log(type(new Person));//"person"

instanceof

instanceof操作符可以确定一个对象是否是特定构造函数的实例

【输出】true或false

【功能】

[1]可以识别内置对象类型、自定义类型及其父类型

[2]不能识别标准类型，会返回false

[3]不能识别undefined、null，会报错

【实例】

console.log("jerry" instanceof String);//false  console.log(12 instanceof Number);//false  console.log(true instanceof Boolean);//false  console.log(undefined instanceof Undefined);//报错  console.log(null instanceof Null);//报错  console.log({name: "jerry"} instanceof Object);//true    console.log(function(){} instanceof Function);//true  console.log([] instanceof Array);//true  console.log(new Date instanceof Date);//true  console.log(/\d/ instanceof RegExp);//true  function Person(){};  console.log(new Person instanceof Person);//true  console.log(new Person instanceof Object);//true