Example of monkey patching Handlebars 'each' method in Meteor to give access to a parent data context inside a child template helper.

parent-data-context.js

if (Meteor.isClient) {

  _.extend(Handlebars._default_helpers, {
    'each': function (data, options) {
      var parentData = this;
      if (data && data.length > 0) {
        return _.map(data, function (childContext, idx) {
          var branch = (childContext._id || (typeof childContext === 'string' ? childContext : null) ||
                        Spark.UNIQUE_LABEL);
          return Spark.labelBranch(branch, function () {
            // won't work if childContext is a string because
            // can't add methods to a string literal.
            if (_.isObject(childContext)) {
              childContext._parentData = parentData || {};
            }
            return options.fn(childContext);
          });
        }).join('');
      } else {
        return Spark.labelBranch('else', function () {
          return options.inverse(parentData);
        });
      }
    }
  });

  Template.list.helpers({
    parentContext: {
      title: "Parent Context"
    },

    items: function () {
      return [{name: "Child Context One"}, {name: "Child Context Two"}]
    }
  });

  Template.listItem.helpers({
    test: function () {
      console.log(this, arguments);
      return "";
    }
  });
}