I'm not sure what your index data format looks like, so imagine the index was in JSON in this format:
{
"items":[
{
"A":[]
},
{
"B":[
"dummy",
"dummy",
"dummy"
]
},
{
"C":[]
},
{
"D":[
"dummy",
"dummy",
"dummy"
]
},
{
"E":[]
},
{
"F":[
"dummy",
"dummy",
"dummy"
]
}
]
}
Now to iterate through displaying used/unused indexes would require a template like:
<ul>
{{#items}}
{{#each .}}
{{#if .}}
<li><a href="#{{@key}}">{{@key}}</a></li>
{{else}}
<li class="unused-letter">{{@key}}</li>
{{/if}}
{{/each}}
{{/items}}
</ul>
which for the above example data would result in:
<ul>
<li class="unused-letter">A</li>
<li><a href="#B">B</a></li>
<li class="unused-letter">C</li>
<li><a href="#D">D</a></li>
<li class="unused-letter">E</li>
<li><a href="#F">F</a></li>
</ul>