Calling a default method in Js Object

Question

Is there any way to create an object that respond to any message? Suppose you have the following object:

function Dog();
Dog.prototype.speak(){
   alert("woof woof");
}
var myDog = new Dog();

Then when you do myDog.speak() you will get an alert with "Woof woof". But what I want is when you call myDog.jump() (which is not defined in the class) you will get a default action like show the user an alert with "you are trying to excecute an inexistent method".

Do you know how can I do it?

See this answer involving `__noSuchMethod__`: http://stackoverflow.com/a/994406/603003 (this is non-standard!) — ComFreek, Oct 23 '13 at 14:36
There is no other way. If I may ask, why do you need such functionality? — ComFreek, Oct 23 '13 at 14:42

score 3 · Answer 1 · answered Oct 23 '13 at 14:44

Short answer: you can't.

Long answer: you could use __noSuchMethod__ but it's not standard and there are some plans to remove it, because Proxy can do the same, and more. Plus, it's a standard. Therefore, you could use a Proxy to do that, but I would discourage to have all objects as proxies, because performance reasons.

Personally, I would just leave the language thrown it's own exception, that the developer can check in the error console.

score 1 · Answer 2 · answered Oct 23 '13 at 14:38

1

There is no standards-based way to do this. The closest thing is this.

The closest you could get to this would be:

function execute(obj, message, args) {
    if (obj[message] && typeof(message) === function) {
        obj[message].call(obj, args);
    } else {
       obj[message] = function() {
           //missing method functionality a la Ruby here
       };
    }
}

answered Oct 23 '13 at 14:38

Mike Thomsen

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ComFreek just said that...in his comment ^^^ – abc123 Oct 23 '13 at 14:40
It should be `typeof obj[message] === 'function'`, shouldn't it? (Also add the quotes in case someone defines the variable `function`.) – ComFreek Oct 23 '13 at 14:44
I don't think you can redefine `function` like you can with `undefined` but other than that you're correct; typeof always returns a string. – Marcus Stade Oct 23 '13 at 15:00

score 0 · Answer 3 · answered Oct 23 '13 at 17:22

Others have already mentioned __noSuchMethod__ and Proxy so I'll refrain from going into further detail on those.

Instead, I wanted to highlight another technique that may be able to do what you want. Please be aware that this is a ugly hack, I can't encourage it's usage and it may not even work in all of your targets. With those caveats in mind, I present you with window.onerror:

window.onerror = function(err) {
  if (/has no method/.test(err)) {
    console.log('oh my: ' + err) // This is where you'd call your callback
    return true
  }

  return false
}


;(function() {
  this.foo() // will be caught by window.onerror
})()

This – at least in my very limited testing – catches TypeErrors (in Chrome at least, mileage may vary) that signified that the method could not be found. Here are some of the reasons why you should not do this:

window.onerror can only have one handler; if your handler is overwritten this won't work
It catches TypeErrors globally, not just for a specific object; i.e. lot's of false positives
It'll make it fun to debug for anyone coming in not knowing where to find this handler
It tightly couples any bit of code you have that relies on this behavior (bad, bad, bad!)

I don't think I can stress enough how much you really shouldn't be thinking of hacking this in. Use Proxy if you can, admit defeat if you can't.

Calling a default method in Js Object

3 Answers3