0.1.5 • Public • Published

Build Status

Deprecation Notice

This transform has been deprecated in favor of the ember-legacy-class-shim, which is overall a better solution to legacy class support in Ember.


This addon adds a transform for using ES Classes with legacy versions of Ember (< 2.13.0). It transforms the class's constructor function into init, which allows both the constructor and class fields to work.

Why is this needed?

The reason legacy versions of Ember need this transform lies, at its core, in the double extend which was used for the longest time to inject services and other things. This double extend ends up creating a new class altogether which, due to the way classes are handled internally in Ember, never calls super.

This means that when we define a class using class it's constructor never gets run. A side-effect of this is that class fields, which are assigned in the constructor, do not get assigned. This substantially reduces the usefulness of class syntax and decorators since they rely substantially on class fields working as expected.

So what does this do?

When combined with the class fields transform, this takes classes defined like so:

class Foo {
  bar = 'baz';
  constructor() {
    // do something

And transforms them into this:

class Foo {
  constructor() {
    if (!this.__didInit) {
  init() {
    // do something
    this.bar = 'baz';
    this.__didInit = true;

Wait, why does the constructor still exist and call init if it doesn't work?

It actually does work when you make objects/classes outside of the standard Ember container lifecycle, so just doing Foo.create() for instance. The logic in place ensures that init only gets called once in all cases.

Ok, any other caveats?

init is very similar to constructor, it gets called in the same context at almost the same time. The key difference is that when using classes normally, the subclass's constructor gets called firstmeaning it gets to do any setup it wants before callingsuper.init, on the other hand, only gets called after most of the setup has been done (if extending fromEmber.Object`).

Ultimately, the code flow before calling super in constructor will differ between legacy versions and modern versions of Ember. To avoid this, simply call super as the very first thing in any classes which extend Ember.Object.

class Foo extends Ember.Object {
  constructor(...args) {
    // do some things


  • git clone <repository-url> this repository
  • cd ember-legacy-class-transform
  • npm install


Running Tests

  • npm test (Runs ember try:each to test your addon against multiple Ember versions)
  • ember test
  • ember test --server


  • ember build

For more information on using ember-cli, visit https://ember-cli.com/.



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