Partial Helper

controllers.application.js

import Ember from 'ember';

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

helpers.my-helper.js

import Ember from 'ember';
const { htmlSafe } = Ember.String;

function PartialT(value, params = []) {
  this.value = value;
  this.params = params;
}

PartialT.prototype = htmlSafe('partial-t');

PartialT.prototype.toHTML = function() {
  return this.value;
};

PartialT.prototype.toString = function() {
  return this.value;
};

export default Ember.Helper.extend({
	compute(params, hash) {
    const keyOrPartial = params[0];
    const callParams = params.slice(1);
    let value = '';
    
    if (typeof keyOrPartial === 'string') {
      value = `key: ${keyOrPartial}`;
    } else if (keyOrPartial instanceof PartialT) {
      value = `partial: ${keyOrPartial}, params: [${keyOrPartial.params.concat(callParams).join(', ')}]`;
    }
    
    return new PartialT(`${value}`, callParams); 
  }
});

templates.application.hbs

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

<p>{{my-helper "foobar"}}</p>
<p>{{my-helper (my-helper "barbaz")}}</p>
<p>{{my-helper (my-helper "quux" "inner") "outer"}}</p>
<br>
<br>

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"
  }
}