Purely functional data structure for Todo list application state

todoAppState.js

/**
 * todoAppState.js -- Application State for TodoMVC as
 * an immutable record
 */
export default class TodoAppState extends Immutable.Record({
  todoItems: Immutable.Map()  // map from id to TodoItem
}) {
  /**
   * functional item update -- returns a new state with the given item included in the
   * set of todo items.  If there is an existing entry for item.id, the result state
   * will map id to item (functional update).
   */
  addItem(item) {
    const nextTodoItems = this.todoItems.set(item.id,item);
    return this.set('todoItems',nextTodoItems);
  }

  /**
   * functional delete -- returns a new state with the item for the given id removed
   */
  removeItem(id) {
    const nextTodoItems = this.todoItems.delete(id);
    return this.set('todoItems',nextTodoItems);
  }

  /** An Immutable.Seq of all todo items */
  getAll() {
    return this.todoItems.toSetSeq();
  }

  /* returns true iff all items are complete */
  areAllComplete() {
    return this.todoItems.every((item) => item.complete);
  }
}