Map, Set, and keyed collections

What this means

Map stores key-value pairs and Set stores unique values. Weak variants allow garbage collection of object keys.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

These collections communicate intent better than forcing every data problem into an object or array.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

const unique = new Set([1, 1, 2]);

Common mistake

Do not use map, set, and keyed collections only because it looks advanced. Start from the problem it solves.

Practice task

Change the example, predict the result, then explain the behavior in your own words.

Remember this

Choose a collection based on lookup, uniqueness, ordering, and lifetime needs.