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]

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?

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,arg2,arg3) {...}

myFunc.call(myObj,arg1,arg2,arg3);
myFunc.apply(myObj,[arg1,arg2,arg3]);

/L

 

[Andy Fish]

Thanks, I'd missed that one

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

 

[Vjekoslav Begovic]

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?

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

 

[Martin Honnen]

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?

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]()

 

[Andy Fish]

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.

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?

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

 

[Lasse Reichstein Nielsen]

For the full explanation, you will have to read the ECMA 262
specification[1]

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

/L

 

[Andy Fish]

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.

Not really. Brackets are only grouping, not meaningful

<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