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. This is 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, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.
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); // ErrorWhen 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.
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Frequently Asked Questions
Is the “Arrays & ReadonlyArray” lesson free?
Yes — the full text of “Arrays & ReadonlyArray” is free to read here on the web. 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. The TypeScript Academy course includes 3 lessons in total.
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, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 3.
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
- Arrays & ReadonlyArray
- Tuples & labeled tuples
- Enums vs const enum vs union-literal alternatives