Mixin behaviour

controllers.application.js

import Ember from 'ember';
import A from 'app/mixin/a';
import B from 'app/mixin/b';

export default Ember.Controller.extend(A, {
  appName: 'Ember Twiddle',
  
  value: 'nothing', 
  
  foo() {
    this._super();
  },
  
  actions: {
    changeValue() {
      this.foo();
    }
  }
}, B);

mixin.a.js

import Mixin  from "@ember/object/mixin";

export default Mixin.create({
  foo() {
    this.set('value', 'a'); 
  }
});

mixin.b.js

import Mixin  from "@ember/object/mixin";

export default Mixin.create({
  foo() {
    this._super();
  }
});

templates.application.hbs

<h1>Mixins</h1>
<br>
<br>
Value: {{value}}
<br>
<br>
<br>
<br>
<button {{action 'changeValue'}}>Change Value</button>
<br>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}