This allows you to create a prototype javascript template, but gives you one extra parameter where you can define default values for anything not passed by the user to .evaluate();

prototype_template_with_default.js

/**
 * Create a new wrapper to the Prototype Template class to
 * allow default values for the template
 *
 * @param template
 * @param defaults
 */
var TemplateWithDefaults = Class.create(Template, {
  initialize: function($super, template, defaults, pattern) {
    this.defaults = defaults || {};
    $super(template, pattern);
  },
  evaluate: function($super, object) {
    // Clone the defaults for this instance to preserve our actual
	  // defaults across evaluations of this template
    var defaults = Object.clone(this.defaults);
    return $super(Object.extend(defaults, object || {}));
  }
});


// Example Usage:
var myTemplate = new TemplateWithDefaults('Hi There, #{name}!', {name: "Person"});
console.log(myTemplate.evaluate());
console.log(myTemplate.evaluate({name: "Paul"}));