Index signatures and dynamic object keys in TypeScript

Index signatures

When an object's keys are dynamic but all values share a type, use an index signature:

interface StringMap {
  [key: string]: string;
}

const headers: StringMap = {
  "Content-Type": "application/json",
  "Authorization": "Bearer abc123"
};

headers["X-Custom"] = "value"; // valid
headers["count"] = 42;        // Error: number not assignable to string

Mixing fixed and dynamic keys

interface Config {
  version: number;
  [key: string]: string | number; // must include number because version is number
}

Record utility type

Record<K, V> is a cleaner shorthand for common index signature patterns:

const scores: Record = {
  alice: 95,
  bob: 88
};

// Restrict keys to a union
type Role = "admin" | "user" | "guest";
const permissions: Record = {
  admin: true,
  user: false,
  guest: false
};

Prefer Record for known key sets and index signatures for fully dynamic keys.