Arbitrary object methods and properties in JavaScript

Question

I'm sure this has definitively been answered before, and I've tried to search for it.. maybe my search terms are wrong...

Basically I have an object myObject, and I have a set of defined properties and methods for it. What I want to do is be able to handle calls/references to properties and methods that I have not defined.

For example, let's say I have this:

var myObject = {
  someProperty : 'foobar',
  someFunction : function () { /* Do stuff */ }
}

Currently, if someone tries to make a call to myObject.someOtherFunction(), JavaScript yells and screams about it. What I want to do is setup a way to automatically handle that. So for example, instead of JavaScript throwing an error, my object just returns false. Is this possible?

Another way to look at it is this:

var myObject = {
  someFunction : function () { /* Do stuff */ }
  magicBucket : function () { /* Do stuff */ }
}

If I call myObject.someFunction(), well that is defined and does something. What I want to happen is if I were to for instance call myObject.someOtherFunction(), instead of JavaScript throwing an error, it would call myObject.magicBucket().

The reason is that I have a client that uses a third-party library on their site. They want to discontinue using it, but completely removing it is going to take a lot of time and effort. So as a short-term solution, they wanted to know if I could make a dummy file that basically does nothing. Well, this library uses several objects that has lots of methods. I could go through everything and make dummy objects, but I thought maybe there might be some easy "catch-all" method to do this.

Some have mentioned checking if the method exists first, wrapping it in a condition or try..catch, etc. Well, the point of this is that at this time I can't touch the actual calls to the methods. And since the overall goal is to eventually remove the coding altogether, it's not even applicable.

This is quite irregular, may I ask why you want this functionality? — azz
– azz, Commented Aug 28, 2013 at 14:37
If you think someOtherFunction will either be undefined or a function, you could do myObject.someOtherFunction?myObject.someOtherFunction():false, so you only invoke myObject.someOtherFunction if it is not a "falsy" value (false undefined, null, 0), or yield false as a fallback value. — apsillers
– apsillers, Commented Aug 28, 2013 at 14:39
This is more or less a reason for try/catch blocks or for checking if the desired function/property exists on the object. If someone other than you is using your code, then you cannot be entirely responsible for them trying to use a potentially nonexistent property. Are you encountering a situation where your object may or may not have someOtherFunction on it, and you want to handle the situation if it does or does not have it? — ajp15243
– ajp15243, Commented Aug 28, 2013 at 14:41
I think you're going to have to make an object with all the methods defined but empty function bodies. What might speed up that process is using for (var mthd in myObject) { if (typeof mthd == 'function') { to programmatically generate a list of methods to be dummies. (I program quick'ndirty scripts to generate scripts all the time, scriptception basically) — asontu
– asontu, Commented Aug 28, 2013 at 15:21

Aadit M Shah · Accepted Answer · 2013-08-28 15:09:08Z

4

There's a special property called __noSuchMethod__ which does precisely what you just described. However it's a non-standard property. It only works in Firefox. Here's how you use it:

var o = {
    __noSuchMethod__: function (name, args) {
        alert(name); // prints the name of the method
        alert(args); // prints the array of arguments
    }
};

o.abc(1, 2, 3); // OUTPUT: abc 1,2,3

The future however are proxy objects. The following is a short tutorial on proxies: Proxy Tutorial

answered Aug 28, 2013 at 15:09

Aadit M Shah

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2 Comments

slinkhi Over a year ago

yep, this is exactly what I need, except that it needs to work in all browsers :/ +1 for being on the right track though

Max Murphy Over a year ago

Thank you for putting me on the right track. The Proxy syntax has changed since that blog post, so the example there doesn't work. MDN has an up-to date example: developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

Peter Mortensen · Accepted Answer · 2014-02-23 16:28:06Z

2

No, you can't have arbitrary getters in JavaScript. You can test if a function exists before calling it to prevent the error though:

if (myObject.someOtherFunction)
    myObject.someOtherFunction();

Or, better, if you don't know that it's necessarily a function:

if (typeof myObject.someOtherFunction == 'function')
    myObject.someOtherFunction();

edited Feb 23, 2014 at 16:28

Peter Mortensen

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answered Aug 28, 2013 at 14:40

Paul

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2 Comments

Bergi Over a year ago

You can have where Proxies are supported…

Paul Over a year ago

@Bergi That's true. That probably won't help in the OP's situation though.

Max Murphy · Accepted Answer · 2016-08-09 11:17:34Z

2

An update on Proxies, here is an infinite array:

$ var squares = new Proxy([], {get:(target,key) => key*key});
$ squares[2]
4
$ Array.isArray(squares)
true
Unfortunately:
$ squares.length
NaN // Want Infinity.

And a dummy object:

$ x = new Proxy({}, {get:(target,key) => console.error("The computer says no", key)})
$ x.laugh
The computer says no laugh

This latter would help the OP make a dummy object, although it would take a bit of black magic to divine what sort of dummy to return.

An up-to date reference: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Proxy

answered Aug 9, 2016 at 11:17

Max Murphy

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Comments

azz · Accepted Answer · 2013-08-28 15:01:46Z

0

You could create a wrapper function like so:

function callFunction(fn, args) {
   var funct = this[fn];
   return (typeof funct == "function") 
      ? funct.apply(this, args)
      : false;
}

And call with:

callFunction("blah", [1, 2, 3]);
>>> false

An example:

this.foo = function(a, b) { 
  console.log(a); 
  return b;
}
callFunction("foo", [1, 2, 3]);
>>> 1 
>>> 2 # return value

edited Aug 28, 2013 at 15:01

answered Aug 28, 2013 at 14:48

azz

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Comments

Ádám Bozzay · Accepted Answer · 2022-12-15 22:29:54Z

0

With Proxy the following works:

function createPseudoObject() {
  return new Proxy(new Function(), { get() { return createPseudoObject() } })
}

Eg. createPseudoObject().foo.bar.toString() does not throw error and returns undefined. By using new Function() as a base for the dummy object I can reference not just any property, but call any function on the dummy object.

Maybe there are some edge cases that are not covered by this, but something like this should work.

answered Dec 15, 2022 at 22:29

Ádám Bozzay

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