invoking a method when I have it's name as a string

[Andy Fish]

Hi,

I'm trying to figure out how to invoke a method on an object given the
object and the method name. For example, here's a simple object with a
method:

function MyObj (pName, pDesc) {
this.name = pName;
this.desc = pDesc;
MyObj.prototype .foo = function() {
return this.name + " - " + this.desc;
}
}

and here is some sample code to manipulate it.

var o1 = new MyObj("nn", "dd");
var s1 = o1.foo();

The assignment to s1 invokes the foo method and gets back the string "nn -
dd" as expected. now I try to find the foo method by it's name:

var funcName = "foo";
var theFunc = o1[funcName];
var s2 = theFunc();

the assignment to s2 invokes the function OK, but not in the context of the
object. can someone tell me how to invoke a function as a method?

Andy

[Lasse Reichstein Nielsen]

Re: invoking a method when I have it's name as a string

"Andy Fish" <ajfish@blueyon der.co.uk> writes:
[color=blue]
> The assignment to s1 invokes the foo method and gets back the string "nn -
> dd" as expected. now I try to find the foo method by it's name:
>
> var funcName = "foo";
> var theFunc = o1[funcName];
> var s2 = theFunc();
>
> the assignment to s2 invokes the function OK, but not in the context of the
> object. can someone tell me how to invoke a function as a method?[/color]

theFunc.call(o1 )

There are two methods of function objects that allow them to pretend to
be methods: "call" and "apply". The only difference is in how they transfer
the arguments to the function. "Call" takes the arguments one at a time,
and "apply" takes them as one array (and it is a type error if the second
argument of "apply" isn't an array).

function myFunc(arg1,arg 2,arg3) {...}

myFunc.call(myO bj,arg1,arg2,ar g3);
myFunc.apply(my Obj,[arg1,arg2,arg3]);

/L
--
Lasse Reichstein Nielsen - lrn@hotpop.com
Art D'HTML: <URL:http://www.infimum.dk/HTML/randomArtSplit. html>
'Faith without judgement merely degrades the spirit divine.'

[Andy Fish]

Re: invoking a method when I have it's name as a string

Thanks, I'd missed that one

hey, this javascript stuff ain't so bad after all... :-)

"Lasse Reichstein Nielsen" <lrn@hotpop.com > wrote in message
news:fzk56bcb.f sf@hotpop.com.. .[color=blue]
>
> theFunc.call(o1 )
>
> There are two methods of function objects that allow them to pretend to
> be methods: "call" and "apply". The only difference is in how they[/color]
transfer[color=blue]
> the arguments to the function. "Call" takes the arguments one at a time,
> and "apply" takes them as one array (and it is a type error if the second
> argument of "apply" isn't an array).
>
> function myFunc(arg1,arg 2,arg3) {...}
>
> myFunc.call(myO bj,arg1,arg2,ar g3);
> myFunc.apply(my Obj,[arg1,arg2,arg3]);
>
> /L
> --
> Lasse Reichstein Nielsen - lrn@hotpop.com
> Art D'HTML: <URL:http://www.infimum.dk/HTML/randomArtSplit. html>
> 'Faith without judgement merely degrades the spirit divine.'[/color]

[Vjekoslav Begovic]

Re: invoking a method when I have it's name as a string

"Andy Fish" <ajfish@blueyon der.co.uk> wrote in message
news:t6p_a.152$ fb1.1755226@new s-text.cableinet. net...[color=blue]
> Hi,
>
> I'm trying to figure out how to invoke a method on an object given the
> object and the method name. For example, here's a simple object with a
> method:
>
> function MyObj (pName, pDesc) {
> this.name = pName;
> this.desc = pDesc;
> MyObj.prototype .foo = function() {
> return this.name + " - " + this.desc;
> }
> }
>
> and here is some sample code to manipulate it.
>
> var o1 = new MyObj("nn", "dd");
> var s1 = o1.foo();
>
> The assignment to s1 invokes the foo method and gets back the string "nn -
> dd" as expected. now I try to find the foo method by it's name:
>
> var funcName = "foo";
> var theFunc = o1[funcName];
> var s2 = theFunc();
>
> the assignment to s2 invokes the function OK, but not in the context of[/color]
the[color=blue]
> object. can someone tell me how to invoke a function as a method?[/color]

var funcName = "foo";
var s2 = o1[funcName]();

[Martin Honnen]

Re: invoking a method when I have it's name as a string



Andy Fish wrote:[color=blue]
> Hi,
>
> I'm trying to figure out how to invoke a method on an object given the
> object and the method name. For example, here's a simple object with a
> method:
>
> function MyObj (pName, pDesc) {
> this.name = pName;
> this.desc = pDesc;
> MyObj.prototype .foo = function() {
> return this.name + " - " + this.desc;
> }
> }
>
> and here is some sample code to manipulate it.
>
> var o1 = new MyObj("nn", "dd");
> var s1 = o1.foo();
>
> The assignment to s1 invokes the foo method and gets back the string "nn -
> dd" as expected. now I try to find the foo method by it's name:
>
> var funcName = "foo";
> var theFunc = o1[funcName];
> var s2 = theFunc();
>
> the assignment to s2 invokes the function OK, but not in the context of the
> object. can someone tell me how to invoke a function as a method?[/color]

Once you assign the method to a variable that function is unbound thus
don't do that, simply call the method on the object
var s2 = o1[funcName]()



--

Martin Honnen
http://JavaScript.FAQTs.com/

[Andy Fish]

Re: invoking a method when I have it's name as a string

Hmm, now I'm confused again. how come this works:

var s2 =(o1[funcName])();

works but this doesn't:

var theFunc = o1[funcName];
var s2 = theFunc();

you'll notice I even put o1[funcName] in brackets for the first example,
which surely must force it to be evaluated as an expression which evaluates
to a value which should be able to be stored in a variable. I even tried
this slightly strange construct:

var s2 =(theFunc = o1[funcName])();

and that doesn't work either.

"Vjekoslav Begovic" <vjbegovic@inet .hr> wrote in message
news:bhd847$4hb $1@sunce.iskon. hr...[color=blue]
> "Andy Fish" <ajfish@blueyon der.co.uk> wrote in message
> news:t6p_a.152$ fb1.1755226@new s-text.cableinet. net...[color=green]
> > Hi,
> >
> > I'm trying to figure out how to invoke a method on an object given the
> > object and the method name. For example, here's a simple object with a
> > method:
> >
> > function MyObj (pName, pDesc) {
> > this.name = pName;
> > this.desc = pDesc;
> > MyObj.prototype .foo = function() {
> > return this.name + " - " + this.desc;
> > }
> > }
> >
> > and here is some sample code to manipulate it.
> >
> > var o1 = new MyObj("nn", "dd");
> > var s1 = o1.foo();
> >
> > The assignment to s1 invokes the foo method and gets back the string[/color][/color]
"nn -[color=blue][color=green]
> > dd" as expected. now I try to find the foo method by it's name:
> >
> > var funcName = "foo";
> > var theFunc = o1[funcName];
> > var s2 = theFunc();
> >
> > the assignment to s2 invokes the function OK, but not in the context of[/color]
> the[color=green]
> > object. can someone tell me how to invoke a function as a method?[/color]
>
> var funcName = "foo";
> var s2 = o1[funcName]();
>
>[/color]

[Lasse Reichstein Nielsen]

Re: invoking a method when I have it's name as a string

Lasse Reichstein Nielsen <lrn@hotpop.com > writes:
[color=blue]
> For the full explanation, you will have to read the ECMA 262
> specification[1][/color]

and the missing footnote is
[1] <URL:http://www.ecma-international.o rg/publications/standards/ECMA-262.HTM>

/L
--
Lasse Reichstein Nielsen - lrn@hotpop.com
Art D'HTML: <URL:http://www.infimum.dk/HTML/randomArtSplit. html>
'Faith without judgement merely degrades the spirit divine.'

[Andy Fish]

Re: invoking a method when I have it's name as a string

"Lasse Reichstein Nielsen" <lrn@hotpop.com > wrote in message
news:ptj94rsz.f sf@hotpop.com.. .[color=blue]
> "Andy Fish" <ajfish@blueyon der.co.uk> writes:
>[color=green]
> > Hmm, now I'm confused again. how come this works:
> >
> > var s2 =(o1[funcName])();
> >
> > works but this doesn't:
> >
> > var theFunc = o1[funcName];
> > var s2 = theFunc();[/color]
>[color=green]
> > you'll notice I even put o1[funcName] in brackets for the first example,
> > which surely must force it to be evaluated as an expression which[/color][/color]
evaluates[color=blue][color=green]
> > to a value which should be able to be stored in a variable.[/color]
>
> Not really. Brackets are only grouping, not meaningful
>[/color]
<snip>

OK, I get it now.

I thought that by putting brackets round it, I would force it to be a
"normal" expression (i.e. one with only one value). It all makes sense when
you add in the concept of reference expressions.

I prostrate myself at the master of javascript, sorry ECMAScript.

Andy