What is an index signature? | Interview Question

Q: What is an index signature?

**Target Audience:** Beginners who need to type a dictionary-like object, and anyone who has been bitten by `obj[key]` being `undefined` at run time. **Difficulty:** Medium **TypeScript version:** Every error and output below comes from TypeScript 5.4.5 with `--strict` (two flag-specific errors were produced by adding that one flag, as noted); 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 A hotel has a key cabinet. The manager does not know in advance which guests will arrive, so the cabinet is not labelled "Mr Smith, Ms Jones". It has a single rule printed on the door: **"Every hook holds exactly one room key."** Any name can go on a hook, but whatever hangs there must b

The problem

Question presented to candidate: "Say you count words in a text and store the counts in an object whose keys you cannot know in advance. How do you type that object, and what are the gotchas?"

What a strong answer should cover:

Syntax and meaning: { [key: string]: number } says every string key maps to a number; key types can be string, number, symbol, template literal patterns such as `data-${string}` (4.4+) or unions of these
Rules: named properties must fit the index signature type (TS2411), and a number index type must be assignable to the string one (TS2413), because number keys become strings at run time
The big gotcha: reading a missing key is typed number, not number | undefined, unless noUncheckedIndexedAccess is on (not part of strict)
Run-time traps of plain objects as dictionaries: inherited keys such as constructor and the __proto__ setter; Object.create(null) or Map avoid them
When to use Record<K, V> (same thing for string keys, required keys for literal unions) and when a Map is the better tool

Clarifying questions expected:

Is the set of keys truly open, or a known list (which would call for Record<Union, V>)?
Can keys come from user input (which raises the __proto__ issue)?

Code / implementation expected: Yes — a typed word-count dictionary and a lookup that handles missing keys.

index-signature

Try it yourself

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

Stuck? Get a hint.

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

Solution

Target Audience: Beginners who need to type a dictionary-like object, and anyone who has been bitten by obj[key] being undefined at run time. Difficulty: Medium TypeScript version: Every error and output below comes from TypeScript 5.4.5 with --strict (two flag-specific errors were produced by adding that one flag, as noted); 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

A hotel has a key cabinet. The manager does not know in advance which guests will arrive, so the cabinet is not labelled "Mr Smith, Ms Jones". It has a single rule printed on the door: **"

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9 min read · 10 sections

Run the code

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

TypeScriptThe rules of index signatures, checked by tsc

Practice

TypeScriptPlain object vs Object.create(null) vs Map as a dictionary

Practice

Discussion

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