Handlebars

Handlebars:

Template

{{#person}}
  {{firstname}} {{lastname}}
{{/person}}

Preparation-Script

Handlebars.registerHelper('blockHelperMissing', function(context, options) {
    return "Helper '"+options.name+"' not found. " 
      + "Printing block: " + options.fn(context); 
});

Input

{
  person: {
    firstname: "Yehuda",
    lastname: "Katz",
  },
}

Output

Helper 'person' not found. Printing block:   Yehuda Katz