• Start Date: 2019-12-08
  • Relevant Team(s): (fill this in with the team(s) to which this RFC applies)
  • RFC PR: https://github.com/emberjs/rfcs/pull/560
  • Tracking: (leave this empty)

Adding Equality Operators to Templates

Summary

Add new built-in template {{eq}} and {{neq}} helpers to perform basic equality operations in templates, similar to those included in ember-truth-helpers.

This RFC is a subset of the changes proposed in #388.

Motivation

It is a very common need in any sufficiently complex Ember app to perform some equality operations and often the most convenient place to do it is right in the templates. Because of that, ember-truth-helpers is one of the most installed addons out there, either directly by apps or indirectly by other addons that those apps consume.

The fact that ember-truth-helpers is so popular is a good signal that this it is filling a perceived gap in Ember's functionality.

A second reason is that it might help make Ember more approachable to newcomers that have some experience in other frameworks. Most if not all web frameworks have some way of comparing values in the templates and it's surprising that Ember requires an third party package to perform even the most basic operations.

Detailed design

Add {{eq}} and {{neq}} helpers.

{{eq}}

Binary operation. Throws an error if not called with exactly two arguments. Equivalent of === This is identical to the eq helper in ember-truth-helpers

{{neq}}

Binary operation. Throws an error if not called with exactly two arguments. Equivalent of !== This is identical to the not-eq helper in ember-truth-helpers, except for the name.

This RFC intentionally leaves the implementation details unspecified, those could be implemented in Glimmer VM or in a higher level in Ember itself.

How we teach this

While the introduction of these helpers doesn't introduce new concepts, as helpers like these could be written and in fact were written for a long time, it might affect slightly how we frame some concepts in the guides.

Previously users were encouraged to put computed properties in the javascript file of the components, even for the most simple tasks like negating a comparing two values using computed.eq.

With the addition of these helpers users don't have to resort to computed properties for simple operations, which sometimes forced users to create javascript files for what could have been template-only components.

In addition to documenting the new helpers in the API docs, the Guides should be updated to favour the usage of helpers over computed properties where it makes more sense, adding illustrative examples and stressing out where the definition of truthiness of handlebars differs from the one of Javascript.

Note on Object Equality

We should also add an additional section to the guides or API docs which discusses using object equality in templates. In general, object equality in JavaScript can be tricky. There are times when it makes perfect sense, for instance finding out if an item is the currently selected item in a list:

class MySelect extends Component {
  items = [{ value: 1 }, { value: 2 }, { value: 3 }];

  @tracked selectedItem = this.items[0];

  isSelected(item) {
    item === this.selectedItem;
  }
}

The {{eq}} helper can be used in a similar way in templates:

<select>
  {{#each this.items as |item|}}
    <option selected={{eq item this.selectedItem}}>
      {{item.value}}
    </option>
  {{/each}}
</select>

There are many valid use cases for object equality. However, there are also times when object equality is not guaranteed, especially in cases where it would have been in Classic Ember. Consider this component:

class MyComponent extends Component {
  @computed('foo')
  get someObj() {
    return { foo: this.foo }
  }

  checkEqual() {
    return this.someObj === this.someObj;
  }
}

checkEqual will return true, because @computed caches the object itself, and returns the same object every time unless foo changes. With Ember Octane, though, by default getters are not cached:

class MyComponent extends Component {
  get someObj() {
    return { foo: this.foo }
  }

  checkEqual() {
    return this.someObj === this.someObj;
  }
}

Now, the someObj getter will rerun every time the property is accessed, returning a new object every time. checkEqual will now always return false, since the objects are not equal to each other.

Now, we can do the same thing in a template with eq:

{{eq this.someObj this.someObj}}

And the result depends here on Ember template's caching strategy. Ember only accesses a given property once, and then it caches the result, so this will return true. However, the semantics of template caches are not guaranteed, and in time may change, so relying on object equality in this way is not generally a good pattern.

Even if the semantics do not change, there are still observable ways that users can trigger the getter twice and generate another object. For instance:

class MyComponent extends Component {
  get someObj() {
    return { foo: this.foo }
  }

  get someObjAlias() {
    return this.someObj;
  }
}
{{eq this.someObj this.someObjAlias}}

Overall, the point here is that if users expect an object generated by a getter or helper to remain stable between accesses, such that object equality or state can be valid, then the user should explicitly cache that value themselves. This can be accomplished in a number of ways, one option being the proposed @cached decorator:

class MyComponent extends Component {
  @cached
  get someObj() {
    return { foo: this.foo }
  }
}

Drawbacks

Adding new helpers increases the surface area of the framework and the code the core team commits to support long term.

Alternatives

One alternative path is don't do anything and let users continue to define their own helpers (or install ember-truth-helpers).

Unresolved questions

  • If an app already use ember-truth-helpers, the {{eq}} helper will conflict with the one proposed here. How do we update ember-truth-helpers to make sure the helper of the same name doesn't collide with the built-in one?
  • The inequality helper proposed in this RFC is {{neq}} while the one in ember-truth-helpers is {{not-eq}}. It is worth considering the benefits that keeping the same name might have in helping apps and addon migrate to the built-in helper.