composable helpers

controllers.application.js

import Ember from 'ember';
import joinWith from '../utils/join-with';

export default Ember.Controller.extend({
  firstName: 'Jim',
  lastName: 'Bob',
  
  fullName(): joinWith('', 'firstName', 'lastName')
  
 });
  

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}

utils.join-with.js

import Ember from 'ember';

const { computed, get } = Ember;

export default function joinWith(separator, ...dependentKeys) {
  return computed(...dependentKeys, function() {
    return dependentKeys
      .map((keyName) => get(this, keyName))
      .join(separator);
  });
}

export default joinWith2(separator, ...dependentKeys) {
  return computed(firstKey, secondKey, function() {
    return firstKey + ' ' + secondKey;
  });
}