title: How to add the number with `index` in `hbs` template
body: <p>I am trying to add a number to an existing <code>index</code> value but I am getting an error. How to solve this?</p>
  
  <p>Here is my try: both <code>tabindex="{{index+1}}"</code>, and <code>class="digit{{index+1}}"</code> throw an error.</p>
  
  <pre><code>{{#each  cardDigitField as  |field index|}}
      &lt;input type="number" tabindex="{{index+1}}" min="0" max="9" maxlength="1" value='' 
      onKeyUp={{action "numberEntered" index}} 
      onKeyPress={{action "numberInit" }} class="digit{{index+1}}"&gt;
  {{/each}}
  </code></pre>