[kwamenum86]

The key is now effectively '1,2,3' in the case [1,2,3] . You can iterate using that string and even pass in the array using bracket notation. But when you iterate using for..in the key will be a string not the original array, and you lose that data. The string is now indistinguishable from the subset of strings with comma delimkted data. May that is inconsequential in your app but for some apps it dampens the effectiveness of the construct.

Ain't JavaScript a riot? [I just found an excuse to use snowclone in an HN comment. Pats self on back]

Functions on the other hand work very well because the key is the function definition (call toString on a function). There is no reason for two functions to have the exact same definition in JavaScript.