String index signatures let us restrict the types that an object can contain.

Address defines an interface. AddressBook defines an interface with a string index signature . The compiler will check that all of the properties of an AddressBook are of type Address . For instance, addressConstants is an object that can contain only properties of type Address .

Here it is in the TypeScript playground .

interface Address {
    street: string;
    postalCode: string;
    province: string;
    country: string;
}

interface AddressBook {
    [key: string]: Address;
}

const addressConstants = {
    // allowed
    OWNER: {
        street: "...", 
        postalCode: "...", 
        province: "....", 
        country: "...",
    },
    // forbidden
    // OWNER_NAME: {
    //    firstName: "...", 
    //    lastName: "...",
    // }
} as AddressBook;