JavaScript: Getset objects in collections

Quite often you find yourself in a situation where you have to an object to a collection
or change a certain property. The “normal” is to test via an if statement whether the
object exists or not:

var obj = collection[someId];  
if(!obj) {  
    obj = {};  
    collection[someId] = obj;  
}  
// do something with object  

This works, is readable, but can be annoying if you have to do this often.

The shorthand syntax for setting a default value for, lets say, a parameter is probably
well known:

param = param || 'default';  

Can we use this somehow in this situation too? Yes we can. “Luckily”, the assignment operation
returns the value that was assigned to a variable:

##11.13.1 Simple Assignment ( = )

The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:

  1. Let lref be the result of evaluating LeftHandSideExpression.

  2. Let rref be the result of evaluating AssignmentExpression.

  3. Let rval be GetValue(rref).

  4. Throw a SyntaxError exception if the following conditions are all true:

    • Type(lref) is Reference is true
    • IsStrictReference(lref) is true
    • Type(GetBase(lref)) is Enviroment Record
    • GetReferencedName(lref) is either “eval” or “arguments
  5. Call PutValue(lref, rval).
    6.Return rval.

(Reference: http://ecma262-5.com/ELS5_HTML.htm)

That means we can use this slightly less readable but much shorter version:

var obj = collection[someId] || (collection[someID] = {});