zohaibs-solution.ts

//your error is at  Line 76 (getTodoByID function)
//........ item.ts......... //
class Item {
  //remember to add field declarations:
  //https://www.typescriptlang.org/docs/handbook/2/classes.html#fields
  taskId: number;
  task: String;
  done: boolean;
  public constructor(taskId: number, task: String, done: boolean = false) {
    this.taskId = taskId;
    this.task = task;
    this.done = done;
  }
  public printTask(): void {
    console.log(
      `id = ${this.taskId}, Task: ${this.task}, Completed: ${this.done}`
    );
  }
}

///........ itemCollection.ts ..... ///
class itemCollection {
  private nextId: number = 1;
  items: Item[] = []; //adding field declarations again
  private itemMap = new Map<number, Item>();

  constructor(public userName: String, items: Item[] = []) {
    items.forEach((item) => this.itemMap.set(item.taskId, item));
  }

  public addToDo(task: String) {
    this.itemMap.set(this.nextId, new Item(this.nextId, task));
    this.nextId++;
  }

  public getToDoItems(isComplete: boolean): Item[] {
    let completedItems: Array<Item> = [];
    this.itemMap.forEach((item) => {
      if (item.done == isComplete) {
        completedItems.push(item);
      }
    });
    return completedItems;
  }
  public removeCompletedItems(): void {
    this.itemMap.forEach((item) => {
      if (item.done == true) {
        this.itemMap.delete(item.taskId);
      }
    });
  }

  /*    public taskDoneFIND(taskId: number) : void
    {
        // finding item through lambda function 'find' whose taskId is same as the parameter and marking it done
        this.itemMap.find((Item) => Item.taskId == taskId).done = true
    } 
    */
  public getTodoById(id: number): Item {
    //your error is HERE!
    //if the user attempts to find an unknown todo eg : getTodoById(10000) (assuming this Todo doesnt exist);
    //then Typescript returns an 'undefined' error. In other words, compiler pretends that you don't have an array at all.
    //therefore, I used an OR expression to tell the user that if the todo isn't found,
    // then it will throw an error.
    //solution copied from: https://stackoverflow.com/a/56540673
    //GET function: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/get
    const todoToFind =
      this.itemMap.get(id) || new Item(0, "Item not found!", false);

    return todoToFind;
  }
  public taskDone(taskId: number): void {
    // finding item through lambda function 'find' whose taskId is same as the parameter and marking it done
    //console.log(this.getTodoById(taskId))
    const completedTodo = this.getTodoById(taskId);
    completedTodo.done = true;
    console.log("this item is now complete: ");
    console.log(completedTodo);
  }
  public printAll() {
    this.itemMap.forEach((item) => item.printTask());
  }
}

//....index.ts ...///
let itemColl: itemCollection = new itemCollection("Zohaib");

itemColl.addToDo("Task1");
debugger;
itemColl.taskDone(1);
itemColl.removeCompletedItems();
itemColl.addToDo("Task2");
itemColl.addToDo("Task3");
itemColl.addToDo("Task4");

//console.log(itemColl.getTodoById(3));

console.log(itemColl.getToDoItems(false));