Convert Dynamic-Key JSON Objects to TypeScript Index Signatures

Dynamic Keys in JSON

Some JSON objects use dynamic keys — keys that are not known ahead of time but follow a pattern. Common examples include dictionaries, lookup tables, and locale maps.

Example JSON

{
  "translations": {
    "en": "Hello",
    "ja": "こんにちは",
    "es": "Hola",
    "fr": "Bonjour"
  },
  "userScores": {
    "user_001": 95,
    "user_002": 82,
    "user_003": 71
  }
}

Generated TypeScript

interface Root {
  translations: { [key: string]: string };
  userScores: { [key: string]: number };
}

Index Signature Syntax

An index signature tells TypeScript that an object can have any number of properties as long as the keys and values match the specified types:

{ [key: string]: ValueType }

Using Record<K, V>

The Record utility type is equivalent and often preferred for readability:

interface Root {
  translations: Record<string, string>;
  userScores: Record<string, number>;
}

Narrowing the Key Type

If you know the possible keys, narrow them with a union:

type Locale = "en" | "ja" | "es" | "fr";

interface Root {
  translations: Record<Locale, string>;
}

This gives you compile-time errors when accessing a key not in the union and enables exhaustive checking.

Mixed Structures

Sometimes an object has known properties and dynamic ones:

interface Config {
  version: string;
  [key: string]: string | number;
}

Be careful: the index signature must be a supertype of all named properties. Here, version: string is compatible with string | number, so it works.

When to Use

Use index signatures for lookup tables, dictionaries, i18n maps, and any structure where the set of keys is data-driven rather than schema-driven.