On mouse enter & leave

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  
  text: "---",
  
  mouseEnter() {
    this.set('text', '+++');
  },
  mouseLeave() {
    this.set('text', '---');
  }  
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}
{{my-component}}

templates.components.my-component.hbs

<div style="padding: 20px; background: #eee; margin: 0 0 5px;">
   {{text}}
</div>

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"
  }
}