Data #3 — Map, Set, WeakMap, WeakSet Polyfills

Map and Set solve problems that plain objects and arrays don't handle well. WeakMap and WeakSet enable memory-safe private data. This article implements polyfills for the first two, and explains all four with runnable examples.

Prerequisites: JS Foundations #4 — Prototypes

1. Why We Need Map — The Problems with Objects as Dictionaries

Plain objects have key limitations: keys are always strings (or Symbols), and inherited properties collide with data:

Map fixes this: any value (object, function, number) can be a key, and keys don't collide with inherited properties.

2. Implementing a Simple Map Polyfill

3. Set — Unique Values

A Set stores unique values — duplicates are ignored:

4. WeakMap — Garbage-Collectable Keys

WeakMap keys must be objects, and they're held weakly — if nothing else references the key, the entry is garbage collected. You can't iterate over a WeakMap:

The main use case: storing private data associated with an object without preventing its garbage collection. This is how #private fields are polyfilled.

5. WeakSet — Weak Collection of Objects

Like WeakMap, WeakSet only stores objects weakly — no iteration, no size:

6. Quick Comparison

Key Takeaways

• Map allows any type as keys and preserves insertion order.
• Set stores unique values — ideal for deduplication.
• WeakMap holds objects weakly — use for private data tied to an object's lifetime.
• WeakMap and WeakSet are not iterable and have no size — by design.
• WeakMap is the ES6 replacement for Object.create(null) with stronger guarantees.

Next: Prototypes #2 — Object.create() & instanceof Polyfills — implement both from scratch.