javascript四种类型识别的方法

前面的话

  javascript有复杂的类型系统,类型识别则是基本的功能。javascript总共提供了四种类型识别的方法,本文将对这四种方法进行详细说明

 

typeof运算符

  typeof是一元运算符,放在单个操作数的前面,返回值为表示操作数类型的首字母小写的字符串

  [注意]typeof运算符后面带不带圆括号都可以

console.log(typeof 'a');//'string'
console.log(typeof ('a'));//'string'

识别

  【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)

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"

  [注意]判断一个值是否为null类型的最佳方法是直接和null进行恒等比较

console.log(typeof null);//'object'
console.log(null === null);//true
console.log(undefined === null);//false
console.log('null' === null);//false

 

instanceof运算符

  instanceof是一个二元运算符,左操作数是一个对象,右操作数是一个构造函数。如果左侧的对象是右侧构造函数的实例对象,则表达式返回true;否则返回false

  如果左操作数不是对象,返回false,如果右操作数不是函数,则抛出一个类型错误异常TypeError

console.log(123 instanceof function(){});//false
//Uncaught TypeError: Right-hand side of 'instanceof' is not an object
console.log({} instanceof 123);

  [注意]所有的对象都是Object的实例

识别

  【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

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

 

constructor属性

  实例对象的constructor属性指向其构造函数。如果是内置类型,则输出function 数据类型(){[native code]};如果是自定义类型,则输出function 数据类型(){}

识别

  【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,因为它俩没有构造函数

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(){}

  可以将constructor属性封装成一个类型识别方法

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

 

Object.prototype.toString()方法

  对象的类属性是一个字符串,用以表示对象的类型信息。javascript没有提供设置这个属性的方法,但有一种间接方法可以查询它。Object.prototype.toString()方法返回了如下格式的字符串:[object 数据类型]

识别

  【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

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]

  可以将Object.prototype.toString()方法封装成一个类型识别方法

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"

  [注意]如果是包装对象,Object.prototype.toString()方法将返回其原始类型

console.log(Object.prototype.toString.call(new Number(123)));//[object Number]
console.log(Object.prototype.toString.call(123));//[object Number]
console.log(Object.prototype.toString.call(new String('abc')));//[object String]
console.log(Object.prototype.toString.call('abc'));//[object String]
console.log(Object.prototype.toString.call(new Boolean(true)));//[object Boolean]
console.log(Object.prototype.toString.call(true));//[object Boolean]

 

posted @ 2016-08-06 17:20 小火柴的蓝色理想 阅读(...) 评论(...) 编辑 收藏