JavaScript與一般的OO語言相比,缺少了一個擷取對象類名的內建函數(如c#的GetType)。而JavaScript內建的typeof函數在判斷數組和對象時,只會返回一個“object”,當然,對使用者自訂的類型也是如此,下面的代碼闡明了這一點:
Code
1functionMyClassA(){
2}
3
4varobj=newObject();
5vararr=newArray();
6varmyobj=newMyClassA();
7
8document.write(typeof(obj)+"<br/>");
9document.write(typeof(arr)+"<br/>");
10document.write(typeof(myobj)+"<br/>");
11
上面的代碼將會輸出:
object
object
object
然而,JavaScript 對象還有一個constructor 屬性(除了某些內建的對象,如window,document之外都有),它指向對象的構造器(constructor),而對象的構造器名字與對象的類型名是一樣的,而構造器的名字又可以從constructor 屬性的字串中被解析出來。下面的代碼就是使用了這種機制,來獲得一個現有對象的類名稱,傳回值為已定義的類名或undefined
Code
1/*Returnstheclassnameoftheargumentorundefinedif
2 it'snotavalidJavaScriptobject.
3*/
4functiongetObjectClass(obj){
5 if(obj&&obj.constructor&&obj.constructor.toString){
6 vararr=obj.constructor.toString().match(
7 /functions*(w+)/);
8
9 if(arr&&arr.length==2){
10 returnarr[1];
11 }
12 }
13
14 returnundefined;
15}
下面是一些使用getObjectClass 方法和內建的typeof方法的例子,用它們分別測試了各種類型的對象:
Code
1functionMyClass(){
2}
3
4functiontest(type,expr){
5 varobj=eval(expr);
6
7 document.write("<p>"+type+":<b>"+
8 expr+"</b><br/>");
9 document.write("-toString()returns:"+
10 obj.toString()+"<br/>");
11 document.write("-typeof()returns:<b>"+
12 typeof(obj)+"</b><br/>");
13 document.write("-getObjectClass()returns:<b>"+
14 getObjectClass(obj)+"</b></p>");
15}
16
17test("Integer","42");
18test("Boolean","true");
19test("String",""HelloWorld!"");
20test("Function","MyClass");
21test("Regularexpression","/Matchthis!/");
22test("Intrinsicobject","document");
23test("Arrayobject","newArray(1,2,3)");
24test("Dateobject","newDate()");
25test("Objectobject","newObject()");
26test("MyClassobject","newMyClass()");
輸出如下:
Code
Integer:42
-toString()returns:42
-typeof()returns:number
-getObjectClass()returns:Number
Boolean:true
-toString()returns:true
-typeof()returns:boolean
-getObjectClass()returns:Boolean
String:"HelloWorld!"
-toString()returns:HelloWorld!
-typeof()returns:string
-getObjectClass()returns:String
Function:MyClass
-toString()returns:functionMyClass(){}
-typeof()returns:function
-getObjectClass()returns:Function
Regularexpression:/Matchthis!/
-toString()returns:/Matchthis!/
-typeof()returns:object
-getObjectClass()returns:RegExp
Intrinsicobject:document
-toString()returns:[object]
-typeof()returns:object
-getObjectClass()returns:undefined
Arrayobject:newArray(1,2,3)
-toString()returns:1,2,3
-typeof()returns:object
-getObjectClass()returns:Array
Dateobject:newDate()
-toString()returns:TueAug1917:32:34UTC+08002008
-typeof()returns:object
-getObjectClass()returns:Date
Objectobject:newObject()
-toString()returns:[objectObject]
-typeof()returns:object
-getObjectClass()returns:Object
MyClassobject:newMyClass()
-toString()returns:[objectObject]
-typeof()returns:object
-getObjectClass()returns:MyClass
我們可以看到,typeof 方法在數組、對象、自訂類型對象 的測試中,都只能返回“object”,而getObjectClass 方法則會獲得真正的類型名稱