How do you make values immutable in TypeScript? | Interview Question

Q: How do you make values immutable in TypeScript?

**Target Audience:** Developers who want to stop accidental mutation of config, state and shared data, and who need to know which guarantee each tool really gives. **Difficulty:** Easy **TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A museum protects a painting in layers. A sign says "Please do not touch" — polite visitors obey it, but nothing physically stops anyone. A rope barrier is harder to ignore. A sealed glass case actually stops hands reaching the canvas. And even the glass case only protects what is inside it: the frame hanging on the wall outside is still exposed. TypeScript gi

The problem

Question presented to candidate: "How do you make values immutable in TypeScript? Walk me through the tools, and tell me which ones actually stop a value changing at run time."

What a strong answer should cover:

const only stops re-assigning the variable; the object it points to can still be changed
Compile-time tools: readonly properties, Readonly<T>, readonly T[] / ReadonlyArray<T>, ReadonlyMap / ReadonlySet, as const, and a recursive DeepReadonly<T> for nested data
Run-time tool: Object.freeze (typed as Readonly<T>), which is shallow, fails silently in sloppy mode and throws in strict mode; a deepFreeze helper for nested data
The limits: all of the type-level tools are erased and shallow, and readonly object properties do not affect assignability, so a mutable alias can still write
Classes: readonly fields set once in the constructor, #private fields with a getter for computed read-only state
The everyday pattern: update by creating new objects ({ ...p, y: 3 }) rather than writing in place

Clarifying questions expected:

Do we need run-time protection (shared config, untrusted plugins) or is a compile-time guarantee enough?
How deep is the data: a flat config object or nested state such as a Redux store?

Code / implementation expected: Optional — a readonly interface, Readonly<T> and Object.freeze, with one rejected write.

readonly
immutability

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Developers who want to stop accidental mutation of config, state and shared data, and who need to know which guarantee each tool really gives. Difficulty: Easy TypeScript version: Every compiler message and output below was produced with TypeScript 5.4.5 (--strict, target ES2022).

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A museum protects a painting in layers. A sign says "Please do not touch" — polite visitors obey it, but nothing physically stops anyone. A rope barrier is harder to ignore. A sealed glass case actually stops hands reaching the canvas. And even the glass case onl

The solution is hidden so you can try first

10 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptFrom const to Object.freeze: what each layer stops

Practice

TypeScriptDeepReadonly, deepFreeze and readonly class fields

Practice

Discussion

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