JavaScript : use a function on the fly from an Object

Question

Well, i understand my question is a little bit strange and the answer seems obvious ("it's impossible !"), but, as JS is a very open language, i go on anyway :)

Let's say, we have the following code :

function dummy() {
}
var obj = new dummy();
var result = obj.aFunction('a','b');

Obviously, the JS interpretor says :

obj.aFunction is not a function

I understand his anger :) but is there a trick to bypass the interpretor (or something like that) or to create on-the-fly the aFunction function into the obj object before the interpretor evaluates all the stuff ?

I've red all about using dynamic function names and so on (using eval() and other tricks) but that don't solve my (weird) problem ...

Thanks in advance.

EDIT : Well, folks, thanks for your answers, but it's not my problematic.

In fact, i used to code in Java with AOP and what i want is :

Create a Valve (or something like this) that catches all the exceptions
Analyse the exception
if the exception corresponds to my 'no function' error, i create from scratch the function and execute it
I garbage this exception
if it's not the good exception i let it to continue its job

Unfortunatly, You cannot do that in JS. Using an ExceptionHandler is not sufficient because its API is too poor ...

But, Thanks to all ...

How are you telling it what `aFunction()` does? You should give more details as to what you are trying to accomplish. — forgivenson, Jul 23 '14 at 18:02
Define it "on the fly" e.g. `obj.aFunction = function(x,y){....}` before using it — Yuriy Galanter, Jul 23 '14 at 18:03
And what should this created function contain? Should it be empty? The only solution I can imagine is using a proxy. And proxies are not supported yet. — Razem, Jul 23 '14 at 18:03
possible duplicate of [var functionName = function() {} vs function functionName() {}](http://stackoverflow.com/questions/336859/var-functionname-function-vs-function-functionname) — Brett Weber, Jul 23 '14 at 18:03
It's an interpreted language, how can you create a function not on the fly? — Matt Harrison, Jul 23 '14 at 18:13
if the function is not defined ... var result - obj.aFunction ? obj.aFunction("a", "b") : (obj.aFunction = function(sParam1, sParam2) { /*definition*/ })("a", "b"); — Brett Weber, Jul 23 '14 at 18:24
possible duplicate of [Does Javascript have something like Ruby's method_missing feature?](http://stackoverflow.com/q/9779624/1048572) @jcvidal is that what you mean? — Bergi, Jul 23 '14 at 18:28

Brandon Gano · Answer 1 · 2014-07-23T18:11:54.497

There are many ways to achieve this. Here is one:

function dummy() {
}
dummy.prototype.aFunction = function (a, b) {
  alert(a + ', ' + b);
};
var obj = new dummy();
var result = obj.aFunction('a','b');

Here is a working example. Warning: it will pop up an alert box.

http://jsfiddle.net/X4F67/

Here is another, if you don't know the function name at runtime:

var name = 'aFunction';
function dummy() {
}
dummy.prototype[name] = function (a, b) {
  alert(a + ', ' + b);
};
var obj = new dummy();
var result = obj[name]('a','b');

Fiddle: http://jsfiddle.net/X4F67/1/

Note that both of these affect the dummy prototype itself, so all instances of dummy would contain aFunction. If you instead want to work directly on the obj, you could use something like this:

...
var obj = new dummy();
obj[name] = function () { };

score 0 · Answer 2 · answered Jul 23 '14 at 18:31

You can create a function on the fly, your code just has some issues.

First of all, setting:

var result = obj.aFunction('a','b');

Is making the browser think that obj.aFunction is an existing function that you are calling, you have to set the function.

obj.aFunction = function(a,b)
{
   alert(a + ' ' + b);
}.bind(obj);

...then you could state:

var result = obj.aFunction;

If you were to say:

var result = obj.aFunction('a', 'b');

...after declaring obj.aFunction, it would be called at that line and alert right away.

Instead, you can do this:

window['result']('a', 'b');

As for declaring a function on the fly, such as writing out a function as a string and using eval() to evaluate the function and append it to a variable, I'm not sure why it doesnt work. This is what I tried:

obj.aFunction = eval("function(a,b){alert(a + ' ' + b);}.bind(obj)");

And I get:

Uncaught Syntax Error: Unexpected token (

Either way, hope that helps.

JavaScript : use a function on the fly from an Object

2 Answers2