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

Press F12 and open the dev-console to see the log output.