# JavaScript data types and data structures — Keyed collections: Maps, Sets, WeakMaps, WeakSets

> These data structures take object references as keys. {{jsxref("Set")}} and {{jsxref("WeakSet")}} represent a collection of unique values, while {{jsxref("Map")}} and {{jsxref("WeakMap")}} represent a collection of key-value associations. You could implement Maps and Sets yourself. However, since ob

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These data structures take object references as keys. {{jsxref("Set")}} and {{jsxref("WeakSet")}} represent a collection of unique values, while {{jsxref("Map")}} and {{jsxref("WeakMap")}} represent a collection of key-value associations.

You could implement Maps and Sets yourself. However, since objects cannot be compared (in the sense of &lt; "less than", for instance), neither does the engine expose its hash function for objects, look-up performance would necessarily be linear. Native implementations of them (including WeakMaps) can have look-up performance that is approximately logarithmic to constant time.

Usually, to bind data to a DOM node, one could set properties directly on the object, or use data- attributes. This has the downside that the data is available to any script running in the same context. Maps and WeakMaps make it easy to _privately_ bind data to an object.

WeakMap and WeakSet only allow garbage-collectable values as keys, which are either objects or non-registered symbols, and the keys may be collected even when they remain in the collection. They are specifically used for memory usage optimization.

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