[thelist] Q on assoc arrays

[Simon Willison]

jsWalter wrote:

> In Perl I can do this...
> 
>   ( 'ac' -> 'Ascension Island',
>     'ad' -> 'Andorra',
>     'ae' -> 'United Arab Emirates',
>     'af' -> 'Afghanistan'
>   )
> 
> Does any one know if JavaScript has a similar "feature"?
> I would rather not have to do...
> 
>    myArray['ac'] = 'Ascension Island';
>    myArray['ad'] = 'Andorra';
>    myArray['ae'] = 'United Arab Emirates';
>    myArray['af'] = 'Afghanistan';

Javascript is actually implemented almost entirely on top of hash 
tables. Objects are hashes, variable scope is handled by hashes, global 
variables are in a hash - if you ever look under the hood of a 
Javascript interpreter most of what it does involves some kind of hash.

The critical point for your purpose is that objects are hashes. A 
Javascript object is simply a data structure that groups a load of names 
with their associated values - it just so happens that these values can 
be code as well as normal data.

The solution to your problem is to use "object literal" syntax, which 
looks like this:

var myArray = {
   ac: 'Ascension Island',
   ad: 'Andorra',
   ae: 'United Arab Emirates',
   af: 'Afghanistan'
};

That will create an object called "myArray" prepopulated with all of 
your data.

Here's an interesting side-note: if you do the above, you'll be able to 
access data from the object using either of the following forms:

var country = myArray['ac'];

or

var country = myArray.ac;

As far as the Javascript interpreter is concerned, they mean the same 
thing. The advantage of the [''] syntax is that it takes a string, which 
you can construct dynamically or obtain from elsewhere in your program. 
You could potentially do the same thing with eval('myArray.'+key) but 
eval is a dirty, dirty hack (and comes with an extra performance hit as 
well).

Hope that helps,

Simon Willison