ember-this-fallback

Polyfills Ember's deprecated Property Fallback Lookup feature to allow apps to continue using it without blocking Ember 4.0+ upgrades.

Compatibility

• Ember.js v3.28.11 or above
• Ember CLI v3.28 or above
• Node.js v16 or above
• ember-cli-htmlbars 6.2.0 or above

Installation

ember install ember-this-fallback

How It Works

The addon registers an ember-cli-htmlbars plugin that traverses the nodes in your Ember templates and transforms them using the following logic:

For each PathExpression with a VarHead that is NOT in the local template scope:

• If it is within node.params or a node.hash value for a CallNode (MustacheStatement | BlockStatement | ElementModifierStatement | SubExpression):

  Prefix the head with this, making it a ThisHead ("expression fallback").

  For example:

  {{! before }}
  {{global-helper property}}
  
  {{! after }}
  {{global-helper this.property}}

• If is the path for a MustacheStatement with NO params or hash:

  • If there is a tail:

    Prefix the head with this, making it a ThisHead ("expression fallback").

    {{! before }}
    {{property.value}}
    
    {{! after }}
    {{this.property.value}}

  • If there is NO tail:

    • If the MustacheStatement is the child of an AttrNode

      • And the AttrNode represents a component argument (the name starts with '@'):

        Prefix the head with this, making it a ThisHead ("expression fallback"), as shown above.

      • And the AttrNode represents an attribute (the name does not start with '@'):

        Wrap the invocation with the tryLookupHelper helper to determine if it is a helper at runtime and fall back to the this property if not ("ambiguous attribute fallback").

        {{! before }}
        <Parent id={{property}} />
        
        {{! after }}
        {{#let (hash property=(tryLookupHelper "property")) as |maybeHelpers|}}
          <Parent
            id={{(if
              maybeHelpers.property (maybeHelpers.property) this.property
            )}}
          />
        {{/let}}

    • Otherwise:

      Wrap the invocation with the isComponent helper to determine if it is a component at runtime and invoke it as a component if so. If not, wrap the invocation with the tryLookupHelper helper to determine if it is a helper ad runtime and fall back to the this property if not ("ambiguous statement fallback").

      {{! before }}
      {{property}}
      
      {{! after }}
      {{#if (isComponent "property")}}
        <Property />
      {{else}}
        {{#let (hash property=(tryLookupHelper "property")) as |maybeHelpers|}}
          {{(if maybeHelpers.property (maybeHelpers.property) this.property)}}
        {{/let}}
      {{/if}}

Caveats

Runtime implications

The isComponent and tryLookupHelper helpers have runtime implications that may have performance impacts. Thus, we recommend relying on this addon only temporarily to unblock 4.0+ upgrades while continuing to migrate away from reliance on the Property Fallback Lookup feature.

Embroider compatibility

In the "ambiguous attribute fallback" and "ambiguous statement fallback" cases shown above, we fall back to dynamic resolution at runtime to determine if the contents of the mustache statement point to a helper, a component, or a property on this. This technique is fundamentally incompatible with Embroider "optimized" mode, specifically the staticHelpers and staticComponents configs (and thus, splitAtRoutes), which requires that helpers are resolvable at build-time.

Thus, in these cases, we log a warning to ember-this-fallback-plugin.log. If you wish to use Embroider's "optimized" mode, we recommend manually updating the code in these cases to either:

{{! For a known property on `this`. }}
{{this.property}}

or

{{! For a known global helper. }}
{{(property)}}

or

{{! For a known global component. }}
<Property />

In the future, we could resolve this incompatibility if we had access to Embroider's static resolution.

Deprecations

Because the intent of this library is to allow users to upgrade to Ember 4.0+ while they still in the process of resolving this-property-fallback deprecations, this library includes the this-property-fallback deprecation from Ember 3.x. You can follow the instructions here for handling these deprecations.

Configuration

You can configure this addon under the 'ember-this-fallback' key in the EmberApp constructor options:

"use strict";

const EmberApp = require("ember-cli/lib/broccoli/ember-app");

module.exports = function (defaults) {
  const app = new EmberApp(defaults, {
    //...
    "ember-this-fallback": {
      /**
       * Disable all logging, including debug logging (even with the `DEBUG`
       * environment variable) and logging to `ember-this-fallback-plugin.log`.
       */
      enableLogging: false,
    },
  });
  // ...
};

Contributing

See the Contributing guide for details.

License

This project is licensed under the MIT License.