Readonly Properties

A readonly property may be read but cannot be reassigned through a type that marks it readonly.

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

This is a compile-time access restriction, not an automatic Object.freeze call. Nested objects remain independently mutable unless their relevant properties are also readonly. Choose readonly for contracts that should not be reassigned during normal program operation.

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

type Settings = { readonly id: number; theme: string };
const settings: Settings = { id: 7, theme: "light" };
settings.theme = "dark";
console.log(settings.id);
console.log(settings.theme);

Verified output

7
dark

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.

TypeScript permits changing theme because that field is mutable. The readonly id is read as 7, and the updated theme logs as dark.

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.

settings.id = 9 is a compile-time error. Do not assume readonly makes nested data immutable or is enforced for untyped JavaScript writes.

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.

Add a nested profile field to Settings. Make its name readonly as well, then test which assignments compile.

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 Properties: A readonly property may be read but cannot be reassigned through a type that marks it readonly.

TypeScript permits changing theme because that field is mutable. The readonly id is read as 7, and the updated theme logs as dark.

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