TypeScript Coach

Index signatures

Index signatures allow you to define the shape of objects that may have unknown or dynamically named properties.

This is useful when dealing with objects where property names are not predetermined but share a common value type.

interface Dictionary {
  [key: string]: string;
}
const translations: Dictionary = {
  hello: "Hola",
  goodbye: "Adiós"
}

The

type CurrencyCode = 'USD' | 'EUR' | 'GBP'
interface ExchangeRates {
    [currency: CurrencyCode]: {
        [other: CurrencyCode]: number
    }
}
const todayRates = {
    USD: {
        EUR: 1.4,
        GBP: 1.2
    },
    EUR: {
        GBP: 1.1
    }
}

It is

type UserRoles = Record<string, "admin" | "user">;
const roles: UserRoles = {
  alice: "admin",
  bob: "user",
};