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- Start Date: 2019-12-08 | ||
- Relevant Team(s): (fill this in with the [team(s)](README.md#relevant-teams) to which this RFC applies) | ||
- RFC PR: https://github.com/emberjs/rfcs/pull/560 | ||
- Tracking: (leave this empty) | ||
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# Adding Equality Operators to Templates | ||
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## Summary | ||
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Add new built-in template `{{eq}}` and `{{neq}}` helpers to perform basic equality operations in templates, similar to those included in `ember-truth-helpers`. | ||
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This RFC is a subset of the changes proposed in #388. | ||
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## Motivation | ||
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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](https://github.com/jmurphyau/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. | ||
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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. | ||
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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. | ||
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## Detailed design | ||
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Add `{{eq}}` and `{{neq}}` helpers. | ||
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#### `{{eq}}` | ||
Binary operation. Throws an error if not called with exactly two arguments. | ||
Equivalent of <arg1> === <arg2> | ||
This is identical to the `eq` helper in `ember-truth-helpers` | ||
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#### `{{neq}}` | ||
Binary operation. Throws an error if not called with exactly two arguments. | ||
Equivalent of <arg1> !== <arg2> | ||
This is identical to the `not-eq` helper in `ember-truth-helpers`, except for the name. | ||
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This RFC intentionally leaves the implementation details unspecified, those could be implemented in Glimmer VM or | ||
in a higher level in Ember itself. | ||
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## How we teach this | ||
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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. | ||
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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`. | ||
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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. | ||
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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. | ||
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### Note on Object Equality | ||
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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: | ||
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```js | ||
class MySelect extends Component { | ||
items = [{ value: 1 }, { value: 2 }, { value: 3 }]; | ||
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@tracked selectedItem = this.items[0]; | ||
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isSelected(item) { | ||
item === this.selectedItem; | ||
} | ||
} | ||
``` | ||
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The `{{eq}}` helper can be used in a similar way in templates: | ||
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```hbs | ||
<select> | ||
{{#each this.items as |item|}} | ||
<option selected={{eq item this.selectedItem}}> | ||
{{item.value}} | ||
</option> | ||
{{/each}} | ||
</select> | ||
``` | ||
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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: | ||
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```js | ||
class MyComponent extends Component { | ||
@computed('foo') | ||
get someObj() { | ||
return { foo: this.foo } | ||
} | ||
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checkEqual() { | ||
return this.someObj === this.someObj; | ||
} | ||
} | ||
``` | ||
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`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: | ||
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```js | ||
class MyComponent extends Component { | ||
get someObj() { | ||
return { foo: this.foo } | ||
} | ||
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checkEqual() { | ||
return this.someObj === this.someObj; | ||
} | ||
} | ||
``` | ||
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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. | ||
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Now, we can do the same thing in a template with `eq`: | ||
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```js | ||
{{eq this.someObj this.someObj}} | ||
``` | ||
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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. | ||
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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: | ||
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```js | ||
class MyComponent extends Component { | ||
get someObj() { | ||
return { foo: this.foo } | ||
} | ||
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get someObjAlias() { | ||
return this.someObj; | ||
} | ||
} | ||
``` | ||
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```hbs | ||
{{eq this.someObj this.someObjAlias}} | ||
``` | ||
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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: | ||
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```js | ||
class MyComponent extends Component { | ||
@cached | ||
get someObj() { | ||
return { foo: this.foo } | ||
} | ||
} | ||
``` | ||
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## Drawbacks | ||
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Adding new helpers increases the surface area of the framework and the code the core team commits to support long term. | ||
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## Alternatives | ||
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One alternative path is don't do anything and let users continue to define their own helpers (or install `ember-truth-helpers`). | ||
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## Unresolved questions | ||
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- 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. |