interface Todo {

    id: number;

    text: string;

    completed: boolean;

}

const todo: Todo = {

    id: ,

    text: "Buy milk",

    completed: false

}

// K extends keyof T: K will be the unit types of 'id', 'text', 'completed'

// T[K] is the lookup tells the Typescript the correct return type

function prop<T, K extends keyof T>(obj: T, key: K): T[K] {

    return obj[key];

}

type TodoId = Todo['id'];

type TodoText = Todo['text'];

type TodoCompleted = Todo['completed'];

const id: TodoId = prop(todo, 'id'); // type number

const text: TodoText = prop(todo, 'text'); // type string

const completed: TodoCompleted = prop(todo, 'completed'); // type boolean