This demonstrates how you can use TypeScript to clearly communicate your needs upfront

forthrightUserSaver.ts

import { IUserDbInstance, UserDB } from 'UserDatabase';

interface INewUser {
    firstName: string,
    middleName?: string,
    lastName: string
}

function honestDbMethod(newUserToSave: INewUser, dbInstance: IUserDbInstance){
    // if(newUserToSave.lastName){ // This if statement is unnecessary because you asked for what you need
    //     // this will never happen
    // }
    return dbInstance.save(newUserToSave);
}

honestDbMethod({ firstName: "Madonna" }, UserDB);
// ^ Compliler Error: Property 'lastName' is missing in type '{ firstName: string; }' but required in type 'INewUser'.

I left the UserDatabase.ts file for brevity, but here it is:

export interface IUserDbInstance {
    save: (newUser: INewUser)=> INewUser
}

export const UserDB: IUserDbInstance = {
    save: (input)=> {
        console.log(input);
        return input;
    }
}