Generics and reusable constraints

What this means

A generic introduces a type parameter that callers can fill with a specific type. Constraints describe the minimum shape that parameter must have.

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

Generics power reusable collections, API helpers, hooks, and utilities without losing the relationship between input and output.

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

function first<T>(items: T[]): T | undefined { return items[0]; }

Common mistake

Do not add a generic when a concrete type communicates the function more clearly.

Practice task

Write a generic function that returns the last item of an array.

Remember this

Use generics when one operation must preserve several possible types.