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Check out http://jlongster.com/A-Study-on-Solving-Callbacks-with-JavaS... and the follow up post. It's easy enough to start using generators alongside your existing async solution (callbacks or promises).

Proxies can be used for data-binding, similar to Object.observe.

I've also yet to find a case where I need WeakMaps instead of an Object or Map. All the benefits seem to be a bit too theoretical, but I likely haven't dug into it hard enough.

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I've used a WeakMap for a kind of memoization.

Assume an immutable object may undergo an expensive transform that returns an object, which is then stored in a WeakMap keyed to the immutable. When there are no new references to that object, the garbage collector will clean it up, but otherwise the expensive transform may be avoided with a lookup on the WeakMap.

It provides a pretty nice way to decouple your caching from the rest of the codebase.

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