The "closure pattern" is one of the most serious JavaScript anti-patterns -- it doesn't make much of a difference in tiny scripts, but for any decent-size JavaScript application, it can hurt performance terribly, especially regarding memory use.
For example, benchmarking the creation of 1 million objects, each with 10 member functions, in both the prototypal style and the closure style (in Chrome):
1 million prototypal objects: Brief pause, 32.4 MB
1 million closure objects: Minutes wait, 368.1 MB
For more info, here's Brendan Eich on the subject:
The hope was that since the functions were created and bound within the function literal, the "create" method could simply assign references to the existing functions instead of creating a new function each time (ie, avoid what munificent mentioned).
Funny thing is, even though the last line indicates that the first and last objects' "ten" method is actually shared between them, I'm still getting about 370 MB of memory usage after creating 1 million objects. I am under assumption that the cause of this memory increase is the fact that each one of the million objects has its own local references to each of the 10 functions. When I put the 10 functions inside of the "create" method, memory usage goes up again to roughly 750 MB.
Something that initially confused me was that Eich's use of the term "prototypal" (a pattern for implementing class-based inheritance) was different from Douglas Crockford's use of it (an alternative to class-based inheritance - http://javascript.crockford.com/prototypal.html).
In case anyone else finds this useful, here's how I sorted it out:
- "class-based" & "prototype-based" inheritance (Crockford refers to them as "classical" & "prototypal" inheritance). Different styles of instantiating objects.
- "closure pattern" & "prototypal pattern" (aka "prototypal inheritance" in this context). Different techniques for implementing class-based inheritance.
Underscore and Backbone will both teach you a lot. This chapter from Eloquent Javascript will also help motivate the rationale behind Underscore:
The main advantage of Douglas Crockford's power constructor pattern is that it allows truly private variables or functions. I used to be adamant that this was the right way to go (and I still appreciate the pattern), but then I found that just following the convention of prefixing private members with an underscore worked fine--most people understand that it means "private". My code also started to look a little neater. All of my instance members are actually tied to the 'this' context object, rather being spread between nested functions or local variables declared within the constructors, and public/privileged methods declared on the new object itself.