Saturday, September 13, 2008

method_missing in Javascript

method_missing in Javascript? Well not yet -- but you might be surprised to learn that some Javascript implementations already feature this functionality. For example, it appears that Mozilla based browsers have had __noSuchMethod__ since at least Javascript version 1.5. Also, it looks like this feature is quite possible to become a standard feature of Javascript in the future although not likely for 3.1. Look here for a couple of threads discussing this feature.

What might you do with a feature like this? If you found this page I imagine you already have a few ideas in mind but if not the Ruby on Rails framework contains many examples of this technique. Ruby on Rails uses Ruby's method_missing for all sorts of "meta-programming" magic like dynamic finders such as ActiveRecord::Base#find_by_login_and_password('bob', 'cheese').

So I can't leave you without an example...If your browser is so inclined, the following text box will allow you to enter any call you like against the "example" object and will use __noSuchMethod__ to spit back information about the call.

The code that sets up the "example" instance looks like this:

I, for one, would really like to see a feature like this become a Javascript standard.

Hey, if you are interested in learning more about Javascript I would recommend the following books: Javascript: The Definitive Guide, Pro Javascript Techniques, and Prototype and You Never Knew JavaScript Could Do This!

Oh yeah, one last thing - since Javascript objects are pretty much just hashes this type of method missing technique really only needs hashes to be extended to allow a default value to be specified like you can do with a Ruby Hash. This would result in the possibility of writing code like this:

I'm not sure what type of syntax could be offered if the object didn't begin its life as a Hash.


  1. Is it only Mozilla browsers that implement this functionality right now?

  2. @cyberchestnut: I can find some documentation that says this is supported in the Rhino and Spidermonkey javascript engines. As for each individual browser, I cooked up this page:
    which sets up an object with a __noSuchMethod__ method that changes the background of the page from red to green. I then submitted this page to The results were pretty bad, it looks like neither Safari nor IE support this technique while Firefox has supported it for some time. Oh - Chrome didn't support it either.