Incorrect (tracking entire structures):

import Component from '@glimmer/component';
import { tracked } from '@glimmer/tracking';

class TodoList extends Component {
  @tracked items = []; // Entire array replaced on every change

  addItem = (item) => {
    // Creates new array, invalidates all consumers
    this.items = [...this.items, item];
  };

  removeItem = (index) => {
    // Creates new array again
    this.items = this.items.filter((_, i) => i !== index);
  };
}