Declaring JavaScript variable and then overwriting it multiple times
You can using chaining, move the toLowerCase() to simplify a little and return the last statement directly:

function() {
      var el = {{element}};
      el = (el.innerText || el.textContent);
      return el.charAt(0).toUpperCase() + el.slice(1).toLowerCase();
}


I'm personally not a fan of using a single variable for many purposes (I think it interferes with readability and maintainability) so I would probably do this:

function() {
      var el = {{element}};
      var text = (el.innerText || el.textContent);
      return text.charAt(0).toUpperCase() + text.slice(1).toLowerCase();
}