Index Signatures in TypeScript

// String index signature
interface StringDict {
  [key: string]: string;
}

const translations: StringDict = {
  hello: "Hola",
  goodbye: "Adiós",
};

interface NumberDict {
  [index: number]: string;
}

const arr: NumberDict = ["a", "b", "c"];
console.log(arr[0]); // "a"

interface MixedDict {
  [index: string]: number | string;
  length: number;     // OK: number is in the union
  name: string;       // OK: string is in the union
}

interface ReadonlyDict {
  readonly [key: string]: number;
}

const scores: ReadonlyDict = { alice: 95, bob: 87 };
// scores.alice = 100; // ❌ Error: Index signature is readonly

// Form errors dictionary
interface FormErrors {
  [field: string]: string | undefined;
}

// API headers
interface Headers {
  [header: string]: string | string[] | undefined;
  "Content-Type": string;
  Authorization?: string;
}