Stage: Accepted Start Date: 2020-12-22 Release Date: Unreleased Release Versions: ember-source: vX.Y.Z ember-data: vX.Y.Z Relevant Team(s): Ember.js RFC PR:

Deprecate passing classBinding and classNameBindings as arguments


In Ember today, it is possible to pass classBinding and classNameBindings as arguments to a component when invoked with curly syntax.

{{some-component classNameBindings="foo:truthy:falsy"}}

These arguments are merged into the class attribute on the class, regardless of whether or not the component is a classic component which contains the classNameBindings logic. It is also fully possible to accomplish with template syntax in alternative ways, so this RFC proposes deprecating them.


classBinding and classNameBindings are Classic component APIs for manipulating the class name of the element that wraps the component. These were "merged properties" on classic components historically, which meant that rather than overwriting the values, they would merge into the superclass when extending a class. And for instances of components this was also true, so passing it as an argument into a component instance would add the name bindings to the class, rather than replacing them.

At some point, the ability for merging on the instance itself was removed. In its place, a general template transform was added, which transformed the classBinding and classNameBindings args into the class argument, with the same semantics.

{{some-component class="bar" classNameBindings="foo:truthy:falsy"}}

{{! becomes }}
{{some-component class=(concat "bar " (if "truthy" "falsy"))}}

Note the conversion of the foo string into a path in the local context, something that is very much unexpected in modern Ember applications.

The output here is a bit cumbersome to write, which may be why the original syntax was kept. However, with angle bracket invocation, we can now write this in a much cleaner way:

<SomeComponent class="bar {{if "truthy" "falsy"}}" />

Given these alternatives, it doesn't make sense any longer to continue supporting this transform, especially given that it also affects components which do not even support classNameBindings, such as Glimmer components. Since class in this case is an argument and not an attribute, and since this only affects curly components, it is unlikely that anyone is actually relying on this behavior, but removing this incongruity seems like the best course of action.

Transition Path

In general, users should convert to use angle bracket syntax to invoke their component, and then use standard interpolation within the class attribute of the component. In cases where this is not possible/desired, users can continue using curly invocation with (concat) to pass the value.

How We Teach This

Deprecation Guide

classBinding and classNameBindings can currently be passed as arguments to components that are invoked with curly invocation. These allow users to conditionally bind values to the class argument using a microsyntax similar to the one that can be defined in a Classic component's class body:

import Component from '@ember/component';

export default Component.extend({
  classNameBindings: ['isValid:is-valid:is-invalid']
{{my-component classNameBindings="isValid:is-valid:is-invalid"}}

Each binding is a triplet separated by colons. The first identifier in the triplet is the value that the class name should be bound to, the second identifier is the name of the class to add if the bound value is truthy, and the third value is the name to bind if the value is falsy.

These bindings are additive - they add to the existing bindings that are on the class, rather than replacing them. Multiple bindings can also be passed in by separating them with a space:

  classNameBindings="some.boundProperty isValid:is-valid:is-invalid"

These bindings can be converted into passing a concatenated string into the class argument of the component, using inline if to reproduce the same behavior. This is most conveniently done by converting the component to use angle-bracket invocation at the same time.


  classNameBindings="some.boundProperty isValid:is-valid:is-invalid"


    {{if "bar"}}
    {{if this.some.boundProperty "bound-property"}}
    {{if this.isValid "is-valid" "is-invalid"}}

Note that we are passing in the class attribute, not the class argument. In most cases, this should work exactly the same as previously. If you referenced the class argument inside of your component, however, you will need to pass @class instead.

If you do not want to convert to angle bracket syntax for some reason, the same thing can be accomplished with the (concat) helper in curly invocation.

    (if "bar")
    " "
    (if this.some.boundProperty "bound-property")
    " "
    (if this.isValid "is-valid" "is-invalid")


  • Introduces a minor amount of churn.