What are access modifiers in TypeScript classes? | Interview Question

Q: What are access modifiers in TypeScript classes?

**Target Audience:** Beginners writing their first TypeScript classes, and developers from Java or C# who expect `private` to be enforced at run time. **Difficulty:** Easy **TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict` (target ES2022, so `#private` fields run natively); version facts are cited. > **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. Every compiler message and output was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First An office building has signs on its doors. "Reception — open to everyone." "Staff only." "Manager only." "Do not move this furniture." Everyone who reads and respects the signs stays where they should. But a sign is not a lock: someone who ignores it can walk straight in. TypeScript `public`, `protected`, `priv

The problem

Question presented to candidate: "What access modifiers does TypeScript give you for class members, what does each one allow, and does private actually keep data private?"

What a strong answer should cover:

public (the default), protected (the class and its subclasses) and private (the declaring class only), plus readonly (assign only in the declaration or constructor)
Parameter properties: constructor(private readonly id: string) declares and assigns a field in one step
These modifiers are compile-time only: they are erased, so private fields show up in JSON.stringify, Object.keys and bracket access, and code that ignores tsc can still change them
JavaScript #private fields (supported since TypeScript 3.8) are enforced by the engine at run time; #x in obj is a brand check (TypeScript 4.5)
Details interviewers probe: protected access only through an instance of the accessing class (TS2446), private constructors for singletons and factories, and that private members make classes compare nominally

Code / implementation expected: Optional — a small class hierarchy showing what each modifier allows and what still happens at run time.

access-modifiers

Try it yourself

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

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Solution

Target Audience: Beginners writing their first TypeScript classes, and developers from Java or C# who expect private to be enforced at run time. Difficulty: Easy TypeScript version: Every error and output below comes from TypeScript 5.4.5 with --strict (target ES2022, so #private fields run natively); version facts are cited.

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. Every compiler message and output was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

An office building has signs on its doors. "Reception — open to everyone." "Staff only." "Manager only." "Do not move this furniture." Everyone who reads and respects the signs stays where they should. But a sign

The solution is hidden so you can try first

9 min read · 9 sections

Run the code

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

TypeScriptThe four modifiers, and what still runs after tsc rejects it

Practice

TypeScriptprivate vs #private, protected access and private constructors

Practice

Discussion

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