From a language design perspective, if Javascript objects are simply associative arrays, then why have objects?

I think that you're confusing the purpose of that statement. It is meant to describe what an object is, not that javascript does the same thing two different ways. As far as I know, you can't declare an "associative array" in Javascript.

If you want one, you create an object As far as convenience goes, the question relates to syntax. I would be very unhappy if they got rid of the object "dot" syntax and forced me to use dictionary-style access syntax for everything. But, I don't think that was the point of your question More clarification The way people treat simple javascript arrays is not to be confused with the use of an "associative array", which is effectively a dictionary.

I think that you're confusing the purpose of that statement. It is meant to describe what an object is, not that javascript does the same thing two different ways. As far as I know, you can't declare an "associative array" in Javascript.

If you want one, you create an object. As far as convenience goes, the question relates to syntax. I would be very unhappy if they got rid of the object "dot" syntax and forced me to use dictionary-style access syntax for everything.

But, I don't think that was the point of your question. More clarification The way people treat simple javascript arrays is not to be confused with the use of an "associative array", which is effectively a dictionary.

Objects are more than associative arrays : They can have methods There is some notion of (prototypal) inheritance. Objects, if you see them only as a mean to store data, are associative arrays ; but they are more than just a way to store data ;-) Objects are not arrays, actually -- strictly speaking, it's quite the contrary : Array is a specific kind of Object.

6 JavaScript objects don't have methods per se. They have properties that can refer to functions. It's am important distinction.

Also, JavaScript objects are "associative arrays. " (en.wikipedia. Org/wiki/Associative_array) That's another term for "map" or "dictionary" (e.g. , it doesn't mean the same thing as just "array" which implies numerical indexing).

– T.J. Crowder Mar 30 '10 at 16:25 Plus one for bringing up object prototypes. – justkt Mar 30 '10 at 16:33.

Javascript has really powerful objects and functions, I recommend that you watch these: yuiblog.com/crockford/ and read this oreilly.com/catalog/9780596517748.

Also, JavaScript objects are more than just associative arrays (although they are associative arrays — or less confusingly, "maps" or "dictionaries"). They're also hierarchical: If you have A, you can create a B that inherits all of A's properties (properly inherits, as opposed to copying) via prototype inheritance. That's a pretty good reason for having objects as well.

Sophisticated javascript programmers follow Doug Crockford's (Javascript: the Good Parts) lead and take advantage of Javascript's closures and modularity to build objects that encapsulate their state. If you use Javascript's native objects, you end up with associative arrays that are transparent to their clients. If you have a pointer to the object, you can see all of it's state.

Crockford explains (page 37) how to define objects that hide their state and only reveal info in response to method calls. : var myObject == function() { var value = 0; return { increment: function(inc) { value += typeof inc == 'number'? Inc : 1; }, getValue: function() { return value; } }; }(); myObject is now an opaque object that responds to increment() and getValue() only.

Sophisticated programmers probably don't, except in very limited situations, because of the memory cost involved, which translates (esp. On IE) into slowness. This pattern should never be mentioned without calling out the costs.

Every single instance will get its own copy of those functions. (You can't rely on implementations to reuse the underlying code and just provide a separate execution context. ) This translates into real memory costs, even on highly-optimized browsers like Chrome.

How much? Try 4.5X-5X as much for just three simple properties (and it rises as you add more). – T.J.Crowder Mar 31 '10 at 10:26 @T.J. Crowder--What do you do instead?

Do you define the function, and then use it as a prototype when creating individual objects? – PanCrit Mar 31 '10 at 17:21 1 I use constructor functions and prototypes, yes. It means that all properties of the instance are public, but also means I get true reuse of the functions.

I usually use the module pattern to get private scope for the entire "class" (obviously JavaScript doesn't have classes, but you know what I mean), but not for individual instances, because of the memory cost of the latter. I talk about what I do here blog.niftysnippets. Org/2009/09/… and here blog.niftysnippets.Org/2009/09/… – T.J.Crowder Apr 2 '10 at 7:01.

Since there are no classes in javascript and everything are objects, including functions, for me it seems somewhat natural that the "base object" can be used as a generic container datastructure called "map" or "dictionary" or "associative array". It's not unique feature of javascript that objects can be used as associative arrays; consider following in PHP: $map = (object) null; $map->name = 'value'; unset($map->name); (Just to make it clear: I'm not comparing PHP to javascript. The former makes absolutely not good example of academic languge.).

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