This is all you need to create middleware for assemble.

assemble-middleware-starter.js

module.exports = function (assemble) {
  var middleware = function(params, next) {
    // do stuff
    next();
  };

  // When do you want the plugin to run?
  middleware.event = 'page:after:render';
  return {
    'assemble-middleware-foo': middleware
  };
};