What are enums and what are their trade-offs? | Interview Question

Q: What are enums and what are their trade-offs?

**Target Audience:** Developers deciding how to model a fixed set of values (statuses, roles, directions) and wanting the real behaviour of enums, not folklore. **Difficulty:** Medium **TypeScript version:** Behaviour checked with TypeScript 5.4.5 and `--strict`; the 5.0 and 5.8 changes are cited from release coverage. > **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. Type claims are checked by `tsc` (an `Equal ` line or a `// @ts-expect-error` line), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First A shop has five kinds of membership: Bronze, Silver, Gold, Platinum, Staff. One team prints them on plastic cards with a code number on the back — handy, but now there is a card printer, a lookup table from number to name, and a rule that only official cards count, even if someone writes "Gold" on a piece of

The problem

Question presented to candidate: "What are enums in TypeScript, and would you use them in a new codebase? Compare numeric enums, string enums, const enum, and a union of string literals or an as const object."

What a strong answer should cover:

An enum is one of the few TypeScript features that emits run-time code: a real object, and for numeric enums a reverse mapping from number to name
Numeric enums auto-increment and accept numbers; since 5.0 an out-of-range literal is rejected, but any non-literal number still is not
String enums have no reverse mapping and are opaque: the raw string "ACTIVE" is not assignable to Status, and two enums with the same values are not interchangeable
const enum is inlined by a full tsc build but not by single-file transpilers, and isolatedModules forbids ambient const enums
Alternatives: a union of string literals (zero run-time cost) or an as const object plus a derived union (named constants, easy iteration, run-time validation)
Trade-offs today: erasableSyntaxOnly (TypeScript 5.8) and Node type stripping reject enums because they are not erasable syntax

Code / implementation expected: Optional — define a string enum and the equivalent as const object, and show how each is used and iterated.

enums

Try it yourself

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Solution

Target Audience: Developers deciding how to model a fixed set of values (statuses, roles, directions) and wanting the real behaviour of enums, not folklore. Difficulty: Medium TypeScript version: Behaviour checked with TypeScript 5.4.5 and --strict; the 5.0 and 5.8 changes are cited from release coverage.

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. Type claims are checked by tsc (an Equal line or a // @ts-expect-error line), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

A shop has five kinds of membership: Bronze, Silver, Gold, Platinum, Staff. One team prints them on plastic cards with a code number on the back — handy, but now there is a card printer, a lookup table from number

The solution is hidden so you can try first

11 min read · 11 sections

Run the code

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

TypeScriptNumeric and string enums at run time and in the checker

Practice

TypeScriptAlternatives: a literal union and an as const object

Practice

Discussion

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