Is there such a thing as a catch-all key for a javascript object?

Question

Considering the following javascript example:

var myobj = {   func1: function() { alert(name in this) },
                func2: function() { alert(name in this) },
                func3: function() { alert(name in this) }
}

myobj.func2(); // returns true
myobj.func4(); // undefined function

Is it possible to create a 'catch-all' key for myobj that will get called if there is no key/function defined (as in func4()) while retaining the myobj.functionCall() format?

Really? Care to point to one? If you can find one, I'll delete the question. — user113716, Mar 01 '10 at 17:18
@Pointy - are you saying that you can't imagine how it could be implemented in the language? If so, how about a wildcard key like * ? — user113716, Mar 01 '10 at 17:20
I'm saying that I'm too dumb to imagine how it'd be implemented :-) Also, the semantics seem like they'd be complicated; would you just get one wildcard entry? What if you set its value to the number 37 instead of a function? — Pointy, Mar 01 '10 at 17:23
@patrick: there was a question very much like yours last week... can't find it though... — jldupont, Mar 01 '10 at 17:24
@jidupont - I've found similar questions that offer workarounds that change the format of the function call. That's no problem. This question is more about whether or not a feature exists in the language to either provide a catch-all or wildcard key, or whether there's some tricky way to accomplish the same effect. — user113716, Mar 01 '10 at 17:27
@Pointy - Keys are unique, therefore only one wildcard would be allowed (in my imaginary world). If it didn't contain a function, you would presumably get the same result as you would if you tried to call a function against any other key that doesn't contain one. — user113716, Mar 01 '10 at 17:46
Yes I guess that'd work. Something seems a little kludgey about it, but whatever; it's Javascript, after all :-) — Pointy, Mar 01 '10 at 18:15
@Pointy - **but whatever; it's Javascript, after all :-) ** How true! :o) — user113716, Mar 01 '10 at 18:45

score 39 · Answer 1 · edited Aug 16 '19 at 08:35

39

You can create a JavaScript object with 'wildcard' or 'catch-all' keys using a Proxy and a getter function. Unlike the solutions provided, a Proxy should work in just about any environment, including Node.js

var foo = new Object()

var specialFoo = new Proxy(foo, {
    get(target,name) {
        // do something here
        return name
    }
})

console.log(specialFoo.blabla) // this will output "blabla"

If you want the properties to be callable, simply return a function:

var specialFoo = new Proxy(foo, {
    get(target,name) {
        return function() {
            console.log('derp')
            return name
        }
    }
})


specialFoo.callMe() // this will print derp

Details: documentation on mozilla

edited Aug 16 '19 at 08:35

zypA13510

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answered May 25 '16 at 20:26

Jochem Stoel

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"about any environment", but not IE :( – Christian Apr 11 '18 at 11:56
@Christian You can use a polyfill for proxies in IE. Just include if needed. See more information here: https://github.com/GoogleChrome/proxy-polyfill – Jochem Stoel Apr 12 '18 at 17:28
At least that polyfill does not support the use case: "This means that the properties you want to proxy must be known at creation time." – Christian Apr 13 '18 at 06:51
@Christian Hmm. Really? There is no way to implement a catch all proxy working on IE at all, period? Not on any version? Other polyfills maybe? I would like to know this for sure. Can you confirm? – Jochem Stoel Apr 14 '18 at 11:30
Well, that cite was from the link you provided. I don't know if it would be possible to create another polyfill... at least I have not found one which can do it. – Christian Apr 14 '18 at 16:48
take a look at https://github.com/krzkaczor/babel-plugin-proxy/blob/master/src/runtime.js – Bernardo Dal Corno Jun 27 '18 at 14:07
Maybe that'd be worth a try, but it's documentation states "It is not suitable for production environments because performance impact is huge." – Christian Aug 10 '18 at 09:06

score 15 · Accepted Answer · edited May 23 '17 at 12:02

15

You're looking for __noSuchMethod__:
JavaScript getter for all properties

edited May 23 '17 at 12:02

Community

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answered Mar 01 '10 at 17:25

Joel A. Villarreal Bertoldi

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Hmmm... I tried it `__noSuchMethod__: function() { alert('caught it') },` but it isn't getting called. – user113716 Mar 01 '10 at 17:33
1

Interesting... it is getting called when I use Firefox, but not Safari. – user113716 Mar 01 '10 at 17:36
2

Probably because it must be Gecko-restricted... Too bad, Mozilla addressed this method as "Non-standard". – Joel A. Villarreal Bertoldi Mar 01 '10 at 17:36
Tried it with the args. No change. Still works in FF, but not Safari. – user113716 Mar 01 '10 at 17:37
https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Object/noSuchMethod - at top it says it's "Non-standard". You may have to go with ugly syntax instead. – Matchu Mar 01 '10 at 17:37
2

So the answer is that it is not a standard feature of javascript. Sound right? – user113716 Mar 01 '10 at 17:42
3

Unfortunately, no, it's not part of the JavaScript standard. – Joel A. Villarreal Bertoldi Mar 01 '10 at 17:44

Is there such a thing as a catch-all key for a javascript object?

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