SO#43479639

components.my-input.js

import Ember from 'ember';

export default Ember.Component.extend({
  actions: {
    keyDown:function(e){
      if(e.key>='a' && e.key<='z'){
        let val = e.target.value;
        let start = e.target.selectionStart;
        let end = e.target.selectionEnd;
        let result = val.substring(0, start)+ e.key.toUpperCase()+ val.substring(end, val.length);
        e.target.value = result;
        e.target.selectionEnd = end+1;
        
        this.get('onChange')(result); 
      	return false;
      }
      return true;
    },
    onInputChange(value) {
      this.get('onChange')(value);
    },
  },
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',

  actions: {
    saveInput(value) {
      this.set('a', value);
    },
  },
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{my-input value=a onChange=(action 'saveInput') }}
{{a}}
{{log (concat 'test' a)}}
<br>
<br>

templates.components.my-input.hbs

<input value={{value}} oninput={{action 'onInputChange' value="target.value"}} onkeydown={{action 'keyDown'}}>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}