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Add a note on weak map shim.

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1 parent 38d38b6 commit 8e8e415fdeed684fbe7143db2b431087b49109d4 @paulmillr paulmillr committed Feb 8, 2012
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@@ -22,6 +22,15 @@ Object.getPropertyNames, Object.is, Object.isnt
Math.sinh, Math.tanh, Math.acosh, Math.asinh, Math.atanh, Math.hypot,
Math.trunc
+## WeakMap shim
+It is not possible to implement WeakMap in pure javascript.
+[es6-collections](https://github.com/WebReflection/es6-collections)
+implementation doesn't held values strongly which is critical
+for the collection. es6-shim decided to not include incorrect shim.
+
+WeakMap has very unusual use-case so you probably won't need it at all (
+use simple `Map` instead).
+
## IE8 support
There are some shims that do not work in IE8, because it is simply not
possible to implement them properly:

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