16. Records and Maps

`Record<K, V>` types a plain object used as a dictionary. `Map<K, V>` is the runtime collection class with guaranteed key order and any key type (not just strings).

ts
const balances: Record<string, number> = { "0xabc": 10, "0xdef": 5 };
balances["0xabc"] += 1;

const cache: Map<string, number> = new Map();
cache.set("0xabc", 10);
cache.get("0xabc"); // number | undefined

Prefer `Map` when keys aren't known ahead of time and you need reliable iteration order or non-string keys; prefer `Record`/plain objects for fixed, known shapes such as configuration maps or lookup tables serialized to JSON.

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