How to add the number with `index` in `hbs` template

I am trying to add a number with existing index but getting error. how to solve this?

here is my try : both tabindex="",class="digit" throw error.

            <input type="number" tabindex="" min="0" max="9" maxlength="1" value='' 
                onKeyUp= 
                onKeyPress= class="digit" >