javascript中4種類型識別的方法

typeof

typeof操作符用于返回正在使用值的類型

【輸出】首字母小寫的字符串形式

【功能】

[1]可以識別標準類型(將Null識別為object)

[2]不能識別具體的對象類型(Function除外)

【實例】

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

返回代表該對象的字符串表示

【輸出】[object 數據類型]的字符串形式

【功能】

[1]可以識別標準類型及內置對象類型

[2]不能識別自定義類型

【構造方法】

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

【實例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]

【實例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

構造函數實例擁有指向其構造函數的constructor屬性

【輸出】function 數據類型(){[native code]}或者function 自定義類型(){}

【功能】

[1]可以識別標準類型、內置對象類型及自定義類型

[2]不能識別undefined、null，會報錯

【構造方法】

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

【實例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);//報錯
console.log((null).constructor);//報錯
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(){}

【實例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));//錯誤
console.log(type(null));//錯誤
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操作符可以確定一個對象是否是特定構造函數的實例

【輸出】true或false

【功能】

[1]可以識別內置對象類型、自定義類型及其父類型

[2]不能識別標準類型，會返回false

[3]不能識別undefined、null，會報錯

【實例】

console.log("jerry" instanceof String);//false
console.log(12 instanceof Number);//false
console.log(true instanceof Boolean);//false
console.log(undefined instanceof Undefined);//報錯
console.log(null instanceof Null);//報錯
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