What are index signatures?

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IT Interview Practice · Accepted Answer

An **index signature** describes the type of properties when you don't know their names ahead of time — modeling dictionaries/maps where keys are dynamic. ```ts interface StringMap { [key: string]: number; // any string key maps to a number } const scores: StringMap = { math: 90, science: 85 }; scores.history = 70; // ✅ any string key allowed scores.math; // number scores.unknownKey; // number (TS assumes it exists!) — a caveat ``` ## The safety caveat With a plain index signature, the compiler assumes **every** key exists, so `scores.typo` is typed as `number` even if it's actually `undefined` at runtime. Enabling `noUncheckedIndexedAccess` fixes this: ```ts // with noUncheckedIndexedAccess: true scores.math; // number | undefined → forces you to handle the missing case ``` ## Combining fixed and dynamic keys ```ts interface Config { name: string; // known property [key: string]: string | number; // plus arbitrary extra keys } ``` All named properties must be assignable to the index signature's value type. ## Record is often cleaner ```ts type Scores = Record; // same as the index signature type Roles = Record<"admin" | "user", boolean>; // constrained keys ``` `Record` is the idiomatic shorthand and supports a restricted key union, which a bare `[key: string]` can't. ## Why it matters Index signatures model genuinely dynamic-keyed objects (lookup tables, caches, parsed JSON maps). Know the safety caveat (`noUncheckedIndexedAccess`) and prefer `Record` for readability — and prefer a `Map` when keys are truly open-ended and you want real iteration/size semantics.

The safety caveat

Combining fixed and dynamic keys

Record is often cleaner

Why it matters

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