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