message-2.ts

class Message {
    greeting: string;
    body: string;
    data: number;
}

function doActualStuff(g: Message & { name: string; }): void {
    console.log(g.name);
}

let m: Message = {greeting: "hey", body: "im a message", data: 42};

let g = { 
    ...m,
    name: "Vasya",
    anotherFieldNeededElsewhere: true,
};

doActualStuff(g);