What does as const do? | Interview Question

Q: What does as const do?

**Target Audience:** Developers who have seen `as const` in codebases and want to know exactly what it changes and when to use it. **Difficulty:** Medium **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 You fill in a form at the bank and write your account number in pen. A helpful clerk types it into the system as "a number, about eight digits long". Technically true — but now the system cannot tell your account apart from anyone else's. What you wanted recorded was **exactly this number, and it must not change**. TypeScript's default inference behaves like that clerk. Write `const config

The problem

Question presented to candidate: "What does as const do in TypeScript? Show me what type you get with and without it, and tell me where you would actually use it."

What a strong answer should cover:

as const is a const assertion: it stops literal widening, makes every property readonly and turns array literals into readonly tuples, recursively through the literal
The before/after types: { method: string } vs { readonly method: "GET" }, string[] vs readonly ["a", "b"]
Why it matters: values then fit literal-union parameters, and you can derive types from data with (typeof ROLES)[number] or (typeof Obj)[keyof typeof Obj], an enum alternative
Returning tuples from functions (return [value, setValue] as const)
Limits: it is compile-time only (nothing is frozen), it only applies to literals (TS1355 otherwise), and it is not a type check
Related tools: as const satisfies T to check a shape while keeping literals (TypeScript 4.9), and const type parameters (TypeScript 5.0) to get the same inference at a call site

Code / implementation expected: Yes — a config or list with as const, plus a union type derived from it.

as-const
literal-types

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Solution

Target Audience: Developers who have seen as const in codebases and want to know exactly what it changes and when to use it. Difficulty: Medium 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

You fill in a form at the bank and write your account number in pen. A helpful clerk types it into the system as "a number, about eight digits long". Technically true — but now the system cannot tell your account apart from anyone else's. What you wanted recorded was **exactly this number, and

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10 min read · 10 sections

Run the code

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

TypeScriptWhat as const changes, proved with type checks

Practice

TypeScriptDeriving unions, enum-like objects and as const satisfies

Practice

Discussion

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