Typescript - How to set the value of an object key in a Map

Question

I have a Map which takes a Lecture as Key and an Array of Todos as value.

lecturesWithTodos: Map<Lecture, Todos[]> = new Map<Lecture, Todos[]>();

Now I first set the key of this Map without any value (because I get the Todos later).

student.semester.lectures.forEach((lecture) => {
    this.lecturesWithTodos.set(lecture, []);
});

Now I just would like to set my Todos to its specific Key.

todos.forEach((todo) => {
   this.lecturesWithTodos.get(lecture).push(todo);
});

But at this point it is always saying "Cannot read property 'push' of undefined". I know I can get it done, when I am using a string as Key, but I would like to use my Object instead, because it makes things easier for me later.

Is there a way how to get the get-Method working on this lecture object?

Answer 1

Although it would be difficult to retrieve elements from the map later on, one way to achieve your objective is illustrated below.

---

Assuming that your types are as follows (change them as per your requirement)

interface Todos {
  id: number;
  name: string;
}

interface Lecture {
  id: number;
  name: string;
}

The given data:

 lectures: Lecture[] = [
    { id: 100, name: 'ENG' },
    { id: 200, name: 'PHY' },
    { id: 300, name: 'BIO' }
  ];

  todos: Todos[] = [
    { id: 100, name: 'Add' },
    { id: 100, name: 'Sub' },
    { id: 300, name: 'Mul' }
  ];

Logic to create the map

ngOnInit() {
   this.lectures.forEach(lecture => {
      this.lecturesWithTodos.set(lecture, []);
   });

   this.todos.forEach(todo => {
      const findLecture = this.findById(todo.id);
      const findAllTodos = this.findTodos(todo.id);

      if (findLecture && findAllTodos) {
        // implies that there is a previous todo added earlier
        this.lecturesWithTodos.set(findLecture, [...findAllTodos, todo]);
      } else if (findLecture) {
        // implies that its a new todo being set
        this.lecturesWithTodos.set(findLecture, [todo]);
      }
  });

}

/** Used to find Lecture based on todo id **/
findById(id: number) {
    return Array.from(this.lecturesWithTodos.keys()).find(e => e.id === id);
}

/** Used to find all previous todos based on todo id **/
findTodos(id: number) {
    // todos is a 2D array
    let todos = Array.from(this.lecturesWithTodos.values());

    // convert to 1D array for finding values from it
    return [].concat.apply([], todos).filter(e => e.id === id);
}