> One of the strengths of object literals is that you can use them as dictionaries. obj.foo === obj['foo']. I believe Harmony sets allow you to use any object

> One of the strengths of object literals is that you can use them as dictionaries. obj.foo === obj['foo']. I believe Harmony sets allow you to use any object as a key, whereas using a plain JavaScript Object instance will coerce the key to a string and use that as a key. There is no non-hacky way to do this in JavaScript currently. You either need to us an array (making lookups O(n)), or give objects a UID to use as the key. Even if you're just talking about sets with strings as keys, there are lots of edge cases waiting to bite non-experts. For example, what if you want your set to have a key called "toString"? Every JavaScript object already has that method, so if you naively test "set[key] !== undefined" or "key in set" with "toString" as the key you always get true. Ok, so use "set.hasOwnProperty(key)". Now what if you happen to have a key called "hasOwnProperty" and you mask the builtin function? So really you should be using "Object.prototype.hasOwnProperty.call(set, key)". Additionally, now you're making a function call, which could add significant overhead in tight loops.