On the fly Middleware without editing the callback array

easy.js

var MIDDLEWARE_TYPES = [
"validate",
"create",
"update",
"destroy"
];
var MIDDLEWARE_TIMINGS = [
"before",
"after"
];
function capitalizeFirstLetter(string) {
return string.charAt(0).toUpperCase() + string.slice(1);
}
function easyExtend(collection){
  collection.getMiddleware = function(type){
    if(!(type in collection._callbacks)) throw new Error(type+" not in the middleware");
    return collection._callbacks[type];
  };
  MIDDLEWARE_TIMINGS.forEach(function(time){
    collection[time] = function(type,fn){
      if(MIDDLEWARE_TYPES.indexOf(type) === -1){
        throw new Error(
          "cannot add middleware not in types: "+
          JSON.stringify(MIDDLEWARE_TYPES)
        );
      }
      if(typeof fn != "function"){
        throw new Error(
          "callback supplied for("+
          time+" "+type+
          " is not a function"
        );
      }
      collection._callbacks[time+capitalizeFirstLetter(type)].push(fn);
    };
  });
  return collection;
}