When would I want to use “class” (static) methods or properties in JavaScript?

Question

In JavaScript, why would one want to attach properties directly to the constructor?

var Human = function() {};
Human.specie = "Homo Sapience";

I've got this question after looking at CoffeeScript‘s __extend helper function, which contains, among the lines:

for ( var key in parent ) { 
  if ( __hasProp.call( parent, key ) ) child[key] = parent[key]; 
} 

which copies properties / methods to the subclassed object directly from the constructor object. But why would anybody do that?

Thanks!

Answer 1

(Edit: In its original form, the question asked about attaching properties to classes vs. attaching them to prototypes, so that's what I'm responding to.)

It's really more a matter of convention than anything else. If you write

Human::specie = "Homo sapiens"

(where Human::specie is CoffeeScript shorthand for Human.prototype.specie) then declare jane = new Human, then jane.specie will be "Homo sapiens" (unless you specifically set jane.specie to something else). In this case, that sounds desirable.

But in other cases, having a property shared across a large number of prototypes makes your code harder to understand. Let's say that you have a Logger class with a config object. If you attached that object to the prototype, then you could write code like this:

log = new Logger
log.config.destination = './foo'

This would change the destination of all Logger instances to './foo', because there's only one config object. If you want config to apply to all Logger instances, then you should attach it to the class proper, removing the ambiguity from the code above:

log = new Logger
Logger.config.destination = './foo'

Answer 2

In a game say you have an object called world. However there will only ever be one world in the game. This is theoretically the reason you would do this.