Index Signatures

An index signature types properties whose names are not known in advance, such as a dictionary mapping arbitrary strings to numeric counts.

What you will learn: the purpose of index signatures, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Every explicitly declared property that matches the index key type must also have a compatible value type. Use Record<string, T> as an alternative where appropriate. Unknown keys can still produce undefined at runtime, even when the default index-signature read type is T; enable noUncheckedIndexedAccess for safer reads.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

interface Inventory { [product: string]: number; }
const inventory: Inventory = { pens: 4, books: 2 };
inventory["pens"] += 3;
console.log(inventory["pens"]);
console.log(Object.keys(inventory).sort().join(", "));

Verified output

7
books, pens

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Inventory maps arbitrary string keys to number values. pens begins at 4 and becomes 7. The sorted key list is books, pens.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not assume a missing index returns 0; reading inventory["erasers"] yields undefined at runtime unless you explicitly handle it.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Create a dictionary mapping city names to temperatures. Read a missing city with ?? and a sensible fallback.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Index Signatures: An index signature types properties whose names are not known in advance, such as a dictionary mapping arbitrary strings to numeric counts.

Inventory maps arbitrary string keys to number values. pens begins at 4 and becomes 7. The sorted key list is books, pens.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.