typeof instanceof constructor

 

JavaScript中，与判断变量的类型相关的，有2个运算符：typeof、instanceof，和一个属性constructor。

通过一个测试的例子来说明。使用Boolean、String、Array和一个自己先创建的类，代码如下所示：

<script language="Javascript">

var myBool = new Boolean();
document.write("Boolean.constructor = " + Boolean.constructor + ".<BR>");
document.write("Boolean.constructor() = " + Boolean.constructor() + ".<BR>");
document.write("myBool.constructor = " + myBool.constructor + ".<BR>");
document.write("myBool.constructor() = " + myBool.constructor() + ".<BR>");
if(typeof(myBool) == Boolean){
   document.write("typeof(myBool) = <B>Boolean</B>.<BR>");
}
else{
   document.write("typeof(myBool) = <B>" + typeof(myBool) +"</B><BR>");
}
if(myBool instanceof Boolean){
   document.write("myBool is a instance of <B>Boolean</B>.<BR><BR>");
}
else if(myBool instanceof [Object]){
   document.write("myBool is a instance of <B>Object</B>.<BR><BR>");
}

var myStr = new String("Hello,Shirdrn!!!");
document.write("String.constructor = " + String.constructor + ".<BR>");
document.write("String.constructor() = " + String.constructor() + ".<BR>");
document.write("String.constructor = " + String.constructor + ".<BR>");
document.write("String.constructor() = " + String.constructor() + ".<BR>");
if(myStr.constructor() == String){
   var sc = myStr.constructor();
   if(sc == ""){
    document.write("myStr.constructor() = \"\".<BR>");
   }
}
if(typeof(myStr) == String){
   document.write("typeof(myStr) = String.<BR>");
}
else{
   document.write("typeof(myStr) = <B>" + typeof(myStr) +"</B><BR>");
}
if(myStr instanceof String){
   document.write("myStr is a instance of <B>String</B>.<BR><BR>");
}
else if(myArray instanceof Object){
   document.write("myStr is a instance of <B>Object</B>.<BR><BR>");
}

var myArray = ["姓名：Shirdrn","年龄：26","Email:[e-mail]"];
document.write("Array.constructor = " + Array.constructor + ".<BR>");
document.write("Array.constructor() = " + Array.constructor() + ".<BR>");
document.write("myArray.constructor = " + myArray.constructor + ".<BR>");
document.write("myArray.constructor() = " + myArray.constructor() + ".<BR>");
if(typeof(myArray) == Array){
   document.write("typeof(myArray) = <B>Array</B>.<BR>");
}
else{
   document.write("typeof(myArray) = <B>" + typeof(myArray) +"</B><BR>");
}
if(myArray instanceof Array){
   document.write("myArray is a instance of <B>Array</B>.<BR><BR>");
}
else if(myArray instanceof Object){
   document.write("myArray is a instance of <B>Object</B>.<BR><BR>");
}

 

function Building(){
   var cost = 0;
   this.setCost = function(c){
    cost = c;
   };
   this.getCost = function(){
    return cost;
   };
};
Building.prototype.located = "";
Building.prototype.projectTeam = "山东第一建筑团队";
Building.prototype.style = "European Style";
Building.prototype.planning = function(){
   var pre = "2月1日：建筑工程全面启动。<BR>";
   pre += "2月1日~11月30日：工程建筑时期。<BR>";
   pre += "11月30日~12月31日：工程检测时期。<BR>";
   pre += "1月1日：完美竣工。<BR>";
   return pre;
};

var myBuilding = new Building();
document.write("Building.constructor = " + Building.constructor + ".<BR>");
document.write("Building.constructor() = " + Building.constructor() + ".<BR>");
document.write("myBuilding.constructor = " + myBuilding.constructor + ".<BR>");
document.write("myBuilding.constructor() = " + myBuilding.constructor() + ".<BR>");
if(typeof(myBuilding) == Building){
   document.write("typeof(myBuilding) = <B>Building.<BR><BR>");
}
else{
   document.write("typeof(myBuilding) = <B>" + typeof(myBuilding) +"</B><BR>");
}
if(myBuilding instanceof Building){
   document.write("myBuilding is a instance of <B>Building</B>.<BR><BR>");
}
else if(myBuilding instanceof Object){
   document.write("myBuilding is a instance of <B>Object</B>.<BR><BR>");
}

</script>

运行结果如下所示：

Boolean.constructor = function Function() { [native code] } .
Boolean.constructor() = function anonymous() { }.
myBool.constructor = function Boolean() { [native code] } .
myBool.constructor() = false.
typeof(myBool) = object
myBool is a instance of Boolean.

String.constructor = function Function() { [native code] } .
String.constructor() = function anonymous() { }.
myStr.constructor = function String() { [native code] } .
myStr.constructor() = .
typeof(myStr) = object
myStr is a instance of String.

Array.constructor = function Function() { [native code] } .
Array.constructor() = function anonymous() { }.
myArray.constructor = function Array() { [native code] } .
myArray.constructor() = .
typeof(myArray) = object
myArray is a instance of Array.

Building.constructor = function Function() { [native code] } .
Building.constructor() = function anonymous() { }.
myBuilding.constructor = function Building(){ var cost = 0; this.setCost = function(c){ cost = c; } this.getCost = function(){ return cost; } }.
myBuilding.constructor() = undefined.
typeof(myBuilding) = object
myBuilding is a instance of Building.

通过程序及其运行结果可以得到如下结论：

JavaScript的类的constructor属性通过测试都是如下形式的：

  

function Function() { [native code] } .

而一个类的constructor方法是如下形式的：

function anonymous() { }.

一个类的对象实例的constructor属性，对于JavaScript内部类是如下这样：

function Array() { [native code] } .

但是对于我们自己创建的JavaScript类，它会得到我们在类的构造函数中设置的内容，如Building类的一个对象实例myBuilding的constructor属性如下所示：

function Building(){ var cost = 0; this.setCost = function(c){ cost = c; }; this.getCost = function(){ return cost; }; }.

对于类的对象实例的constructor方法(即constructor())，JavaScript内部类的对象实例都有自己的默认值，比如Boolean的对象实例的是的constructor()值为false，而String的对象实例的是的constructor()值为""，Array的对象实例的是的constructor()值为""；但是对于我们自己创建的类就是undefined的。