What is the Record utility type? | Interview Question

Q: What is the Record utility type?

**Target Audience:** Beginners who have met `Record ` in code and want to know what it really is and when it helps. **Difficulty:** Medium **TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict`; the definition is quoted from the 5.4.5 `lib.es5.d.ts`. > **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 A school prints a **register** for each class. The left column lists every pupil, already printed; the right column has one box per pupil for the same kind of thing: "present" or "absent". The teacher cannot skip a pupil (the empty box stands out) and cannot add a pupil who is not on the list (there is no row for them). `Record ` is that register

The problem

Question presented to candidate: "What does Record<K, V> do in TypeScript? When would you reach for it instead of writing an index signature or an object type by hand?"

What a strong answer should cover:

Record<K, V> builds an object type whose keys are K and whose values are all V; in lib.es5.d.ts it is the mapped type { [P in K]: T } with K extends keyof any
With a union of literal keys every key is required and unknown keys are rejected, which gives exhaustiveness: adding a union member breaks every table that misses it
With string keys it is exactly an index signature, with the same "missing key is still typed V" gotcha
Common patterns: lookup tables of labels or handlers, Partial<Record<K, V>> for optional keys, grouping results with Record<string, T[]>
Limits: Object.keys still returns string[], and plain objects carry prototype keys; Map is better for dynamic, user-supplied keys

Code / implementation expected: Yes — a lookup table keyed by a status union, and what tsc says when a key is missing.

utility-types
record

Try it yourself

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Solution

Target Audience: Beginners who have met Record in code and want to know what it really is and when it helps. Difficulty: Medium TypeScript version: Every error and output below comes from TypeScript 5.4.5 with --strict; the definition is quoted from the 5.4.5 lib.es5.d.ts.

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

A school prints a register for each class. The left column lists every pupil, already printed; the right column has one box per pupil for the same kind of thing: "present" or "absent". The teacher cannot skip a pupil (the empty box stands out) and cannot add a pu

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8 min read · 9 sections

Run the code

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

TypeScriptRecord as a mapped type and an exhaustive lookup table

Practice

TypeScriptRecord<string, V> vs Record<Union, V>, grouping and Partial

Practice

Discussion

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