Readonly Arrays

ReadonlyArray<T> and readonly T[] prevent writes through that statically typed reference. They describe a type-checking rule, not a frozen runtime object.

What you will learn: the purpose of readonly arrays, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Readonly array types still permit safe reading, iteration, and nonmutating operations. A separate mutable reference to the same array can still change the contents. To enforce runtime restrictions, you need an appropriate runtime strategy such as copying and freezing, with attention to shallow versus deep immutability.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

const mutable: number[] = [1, 2];
const view: readonly number[] = mutable;
console.log(view.join(", "));
mutable.push(3);
console.log(view.length);

Verified output

1, 2
3

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

The readonly view initially sees 1,2. mutable is another reference to the same array, so push(3) changes the underlying array and view.length becomes 3.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

view.push(3) would be rejected by TypeScript, but the readonly type itself does not prevent mutations via another reference at runtime.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Copy mutable using [...mutable] and give the copy a readonly type. Compare the results after changing mutable.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Readonly Arrays: ReadonlyArray<T> and readonly T[] prevent writes through that statically typed reference.

The readonly view initially sees 1,2. mutable is another reference to the same array, so push(3) changes the underlying array and view.length becomes 3.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.