Computed Properties and Aggregate Data

Sometimes a computed property needs to update when items are added to, removed from, or replaced in an array. In those cases we can use the [] array key to tell the property to update at the right time. We'll use the familiar todo list for our examples:

import EmberObject, { computed } from '@ember/object';
import Component from '@ember/component';

export default Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      EmberObject.create({ title: 'Buy food', isDone: true }),
      EmberObject.create({ title: 'Eat food', isDone: false }),
      EmberObject.create({ title: 'Catalog Tomster collection', isDone: true }),
    ]);
  },

  titles: computed('todos.[]', function() {
    let todos = this.get('todos');
    return todos.mapBy('title');
  })
});

The dependent key todos.[] instructs Ember.js to update bindings and fire observers when any of the following events occurs:

1. The todos property of the component is changed to a different array.
2. An item is added to the todos array.
3. An item is removed from the todos array.
4. An item is replaced in the todos array.

Notably, the computed property will not update if an individual todo is mutated. For that to happen, we need to use the special @each key.

Sometimes you have a computed property whose value depends on the properties of items in an array. For example, you may have an array of todo items, and want to calculate the incomplete todo's based on their isDone property.

To facilitate this, Ember provides the @each key illustrated below:

import EmberObject, { computed } from '@ember/object';
import Component from '@ember/component';

export default Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      EmberObject.create({ isDone: true }),
      EmberObject.create({ isDone: false }),
      EmberObject.create({ isDone: true }),
    ]);
  },

  incomplete: computed('todos.@each.isDone', function() {
    let todos = this.get('todos');
    return todos.filterBy('isDone', false);
  })
});

Here, the dependent key todos.@each.isDone instructs Ember.js to update bindings and fire observers when any of the following events occurs:

1. The todos property of the component is changed to a different array.
2. An item is added to the todos array.
3. An item is removed from the todos array.
4. An item is replaced in the todos array.
5. The isDone property of any of the objects in the todos array changes.

It's important to note that the @each key can be dependent on more than one key. For example, if you are using Ember.computed to sort an array by multiple keys, you would declare the dependency with braces: todos.@each.{priority,title}

Both [] and @each will update bindings when the array is replaced and when the members of the array are changed. If you're using @each on a particular property, you don't also need to use []:

  //specifying both '[]' and '@each' is redundant here
  incomplete: computed('todos.[]', 'todos.@each.isDone', function() {
  ...
  })

Using @each is more expensive than [], so default to [] if you don't actually have to observe property changes on individual members of the array.

Ember also provides a computed property macro computed.filterBy, which is a shorter way of expressing the above computed property:

import EmberObject, { computed } from '@ember/object';
import { filterBy } from '@ember/object/computed';
import Component from '@ember/component';

export default Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      EmberObject.create({ isDone: true }),
      EmberObject.create({ isDone: false }),
      EmberObject.create({ isDone: true }),
    ]);
  },

  incomplete: filterBy('todos', 'isDone', false)
});

In both of the examples above, incomplete is an array containing the single incomplete todo:

import TodoListComponent from 'app/components/todo-list';

let todoListComponent = TodoListComponent.create();
todoListComponent.get('incomplete.length');
// 1

If we change the todo's isDone property, the incomplete property is updated automatically:

import EmberObject from '@ember/object';

let todos = todoListComponent.get('todos');
let todo = todos.objectAt(1);
todo.set('isDone', true);

todoListComponent.get('incomplete.length');
// 0

todo = EmberObject.create({ isDone: false });
todos.pushObject(todo);

todoListComponent.get('incomplete.length');
// 1

Note that @each only works one level deep. You cannot use nested forms like todos.@each.owner.name or todos.@each.owner.@each.name.

Sometimes you don't care if properties of individual array items change. In this case use the [] key instead of @each. Computed properties dependent on an array using the [] key will only update if items are added to or removed from the array, or if the array property is set to a different array. For example:

import EmberObject, { computed } from '@ember/object';
import Component from '@ember/component';

export default Component.extend({
  todos: null,

  init() {
    this.set('todos', [
      EmberObject.create({ isDone: true }),
      EmberObject.create({ isDone: false }),
      EmberObject.create({ isDone: true }),
    ]);
  },

  selectedTodo: null,
  indexOfSelectedTodo: computed('selectedTodo', 'todos.[]', function() {
    return this.get('todos').indexOf(this.get('selectedTodo'));
  })
});

Here, indexOfSelectedTodo depends on todos.[], so it will update if we add an item to todos, but won't update if the value of isDone on a todo changes.

Several of the Ember.computed macros utilize the [] key to implement common use-cases. For instance, to create a computed property that mapped properties from an array, you could use Ember.computed.map or build the computed property yourself:

import EmberObject, { computed } from '@ember/object';

const Hamster = EmberObject.extend({
  excitingChores: computed('chores.[]', function() {
    return this.get('chores').map(function(chore, index) {
      return `CHORE ${index + 1}: ${chore.toUpperCase()}!`;
    });
  })
});

const hamster = Hamster.create({
  chores: ['clean', 'write more unit tests']
});

hamster.get('excitingChores'); // ['CHORE 1: CLEAN!', 'CHORE 2: WRITE MORE UNIT TESTS!']
hamster.get('chores').pushObject('review code');
hamster.get('excitingChores'); // ['CHORE 1: CLEAN!', 'CHORE 2: WRITE MORE UNIT TESTS!', 'CHORE 3: REVIEW CODE!']

By comparison, using the computed macro abstracts some of this away:

import EmberObject from '@ember/object';
import { map } from '@ember/object/computed';

const Hamster = EmberObject.extend({
  excitingChores: map('chores', function(chore, index) {
    return `CHORE ${index + 1}: ${chore.toUpperCase()}!`;
  })
});

The computed macros expect you to use an array, so there is no need to use the [] key in these cases. However, building your own custom computed property requires you to tell Ember.js that it is watching for array changes, which is where the [] key comes in handy.