Step 3 of 6
Hash maps (objects, dictionaries, Maps, Sets)
A hash map turns a key into a storage location, so finding 'user_123' is basically instant regardless of size. In JavaScript that's an object, Map or Set; in Python a dict or set.
Why it matters when you build with AI: The single most useful fix for slow code: replace array.find() inside a loop with a Map built once. That turns O(n²) into O(n).
Quick check
Best fix for users.find() inside a loop over orders?
Map lookups are ~O(1), turning O(n²) into O(n).