[Javascript] property instead of method

[liorean at gmail.com (liorean)]

On 25/05/06, Steve Clay <sclay@xxxxxxx> wrote:
<http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Guide:Creating_New_Objects:Defining_Getters_and_Setters>
> Is there power in these beyond syntactic sugar?

Yes. They allow the emulation of things like the dynamically updating
length property on arrays, of browser objects that have "magic"
properties (See [IE Emu]) etc.

> The above URL shows how to
> create a dynamic "year" property to Date, and I can see that'd be handy for
> something like "myDate.year += 10" but what could you do that you couldn't
> by adding custom methods getYear and setYear? Can you add functions to
> existing getters/setters?

I would expect them to be overridable, but I don't know how that would
work on {ReadOnly} properties and the like.

> Seems to me that a language feature that disguises methods as properties
> just serves to obfuscate interfaces, but I guess these are already out
> there: Shouldn't Element.getChildNodes be a method if hasChildNodes is? How
> do you test if a property is read-only? You can at least test for the
> existence of a setSomething method.

Test if it's read-only, huh? Well, it's not straight forward to test
for this since you might have encountered a setter so that the actual
vaulue you get from reading it out differs from the value you're
trying to set, it might cast to a certain type, like boolean, or it
might only allow a certain set of values.

var
    result=false,
    temp,
    newValue=/*something that should be a valid value for the property*/;
try{
    temp=o.property;
    o.property=newValue;
    if(o.property===newValue)
         result=true;
}catch(err){
    console.log(err);
}


> I'm sure "dynamic" properties do make our lives easier, it's just annoying
> to have code mysteriously and quietly fail because you tried to set a
> property that was read-only only in certain browsers/moon phases and the
> only way to know is to break out the documentation and start browser
> sniffing...

Well, that's the problem with ECMAScript 3 - it doesn't allow us a
sufficiently deep introspection mechanism for non-native code, and it
doesn't allow us to do the same type of things in native code as
non-native code can do. Getters and setters is a prime example of
this.

[IE Emu] <uri:http://webfx.eae.net/dhtml/ieemu/>
-- 
David "liorean" Andersson
<uri:http://liorean.web-graphics.com/>