Arrays, tuples, and readonly data

What this means

An array type describes repeated values. A tuple describes a fixed number of positions with specific types. Readonly types prevent accidental mutation through that reference.

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

Precise collection types make API results, coordinates, options, and immutable inputs easier to use correctly.

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 topics: string[] = ["types", "functions"];
const result: readonly [string, number] = ["passed", 90];

Common mistake

Do not use a tuple when named object properties would communicate the meaning more clearly.

Practice task

Create a readonly list of lesson names and render it without mutating it.

Remember this

Use arrays for repeated items and tuples only for meaningful fixed positions.