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constructor functions

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Jun 20.2008

[I]
Constructor functions typically do not have return values. They initialize the object passed as the value of this and return nothing. However, a constructor is allowed to return an object value, and, if it does so, that returned object becomes the value of the new expression. In this case, the object that was the value of this is simply discarded.[/I]

Can someone explain what that meant?, and if it is what i think it means, then why isnt it applying to this?(why does it not assign 10 to rect1 and why does it still have the width value) :

[code]
 <html>
<body>
<script>
function rectangle(w,h)
{
this.width=w;
this.height=h;
return 10;
}

var rect1 = new rectangle(10,20);
alert(“rect:”+rect1+”width:”+rect1.width);
</script>
</body>
</html>
[/code]

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JavaScript

6 Comments(s) ↴

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@Declan1991Jun 20.2008 — I thought that the return was ignored actually. Where did you get the definition from?
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@Orc_ScorcherJun 20.2008 — Because 10 is a primitive value, not a object value. Try [b]return new Number(10)[/b] instead.
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@forrestgumpauthorJun 20.2008 — I thought that the return was ignored actually. Where did you get the definition from?[/QUOTE]

OReilly.JavaScript.The.Definitive.Guide.5th.Edition.Aug.2006
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@forrestgumpauthorJun 20.2008 — Because 10 is a primitive value, not a object value. Try [b]return new Number(10)[/b] instead.[/QUOTE]

? Thnx dude....
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@KorJun 20.2008 — In fact the called [B]new[/B] token makes the difference between using a function as a constructor or not:
<i>
</i>function myFunction(){
return 10;
}
function myConstructor(){
return [COLOR="Blue"]new[/COLOR] Number(10);
}

var obj={}
obj.x= myFunction();
obj.y=[COLOR="Blue"]new[/COLOR] myConstructor();
alert(obj.x);
alert(obj.y);
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@NandemJun 20.2008 — In fact the called [B]new[/B] token makes the difference between using a function as a constructor or not:
<i>
</i>function myFunction(){
return 10;
}
function myConstructor(){
return [COLOR="Blue"]new[/COLOR] Number(10);
}

var obj={}
obj.x= myFunction();
obj.y=[COLOR="Blue"]new[/COLOR] myConstructor();
alert(obj.x);
alert(obj.y);
[/QUOTE]


Exactly, the "new" keyword makes a different call to a function, but

remember, the "return" keyword used on a constructor, will ONLY be ignored when the return value is a "ValueType", if the return value is a "ReferenceType" will be returned by the constructor instead:


<i>
</i> [COLOR="Blue"]function[/COLOR] Constructor1()
{
this.a = 1;
this.b = 2;
return 10;
}

<i> </i>[COLOR="Blue"]function[/COLOR] Constructor2()
<i> </i>{
<i> </i> this.a = 1;
<i> </i> this.b = 2;
<i> </i> return [COLOR="Blue"]new[/COLOR] [COLOR="Blue"]Object[/COLOR]();
<i> </i>}
<i> </i>window.onload = [COLOR="Blue"]function[/COLOR]
<i> </i>{
<i> </i> [COLOR="Blue"]var[/COLOR] obj1 = [COLOR="Blue"]new[/COLOR] Constructor1(); [COLOR="SeaGreen"]//&lt;--WILL BE A OBJECT WITH a,b properties[/COLOR]
<i> </i> [COLOR="Blue"]var[/COLOR] obj2 = Constructor1(); [COLOR="SeaGreen"]// &lt;-- WILL BE A VALUETYPE[/COLOR]

<i> </i> [COLOR="Blue"]var[/COLOR] obj3 = [COLOR="Blue"]new[/COLOR] Constructor2(); [COLOR="SeaGreen"]// &lt;-- ALWAYS WILL RETURN A EMPTY OBJECT[/COLOR]
<i> </i> [COLOR="Blue"]var[/COLOR] obj4 = Constructor2(); [COLOR="SeaGreen"]// &lt;-- ALWAYS WILL RETURN A EMPTY OBJECT[/COLOR]
<i> </i>}



cya
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