Polymer.js is a JavaScript library created by Google that allows reusing the HTML elements for building applications with components.
To achieve this index, you need to set the index as:
<span>{{displayIndex(index)}}</span>
The displayIndex would be:
function (index) { return index + 1; }
Let us see it in an example:
<div>Subject <span>{{displayIndex(index)}}</span></div>