Felix Kling

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] = {});