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TypeScript Academy · Lesson

Arrays & ReadonlyArray

Learn array basics, the difference between mutable arrays and ReadonlyArray, and how to use typed methods safely.

Arrays & ReadonlyArray is a free TypeScript Academy lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro

Goal: Understand how to declare arrays, use methods with types, and know when to use ReadonlyArray for immutability.

Array declaration

Arrays can be typed with element type syntax (e.g., number[]).

const numbers: number[] = [1, 2, 3];
console.log(numbers[0]);

Generic Array type

You can also use the generic form Array<T> which is equivalent to T[].

const words: Array<string> = ["a", "b", "c"];
console.log(words.length);

Array methods

Array methods like map, filter, and reduce preserve type information automatically.

const nums: number[] = [1, 2, 3];
const doubled = nums.map(n => n * 2);
console.log(doubled);

ReadonlyArray

ReadonlyArray ensures immutability: no push, splice, or reassign methods are available.

const readonlyNums: ReadonlyArray<number> = [1, 2, 3];
// readonlyNums.push(4); // Error

When to use ReadonlyArray

Use ReadonlyArray when you want to

  • Guarantee no accidental mutation
  • Pass data safely across functions
  • Signal intent of immutability

ReadonlyArray check

Quick check: Which statement about ReadonlyArray in TypeScript is TRUE?

Recap

Recap: Arrays can be typed as T[] or Array<T>. Use methods with type safety. Choose ReadonlyArray when immutability is required.

Frequently asked questions

Is the “Arrays & ReadonlyArray” lesson free?

Yes — the full text of “Arrays & ReadonlyArray” is free to read here on the web, and the TypeScript Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO.

What will I learn in “Arrays & ReadonlyArray”?

Learn array basics, the difference between mutable arrays and ReadonlyArray, and how to use typed methods safely. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Arrays & ReadonlyArray” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Arrays & ReadonlyArray
  2. Tuples & labeled tuples
  3. Enums vs const enum vs union-literal alternatives
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