Handlebars:
Template
{{#person}}
  {{firstname}} {{lastname}}
{{/person}}
Preparation-Script
// Handlebars.registerHelper('loud', function(string) {
//    return string.toUpperCase()
// });
Input
{
  person: {
    firstname: "Yehuda",
    lastname: "Katz",
  },
}
Output
  Yehuda Katz