define namespace

core.js

// namespace
window.MyApp = {};

// define namespace
MyApp.define = function(name, fn) {
  var parent = MyApp;
  var parts = name.split('.');

  parts.forEach(function(part, i) {
    var isLast = parts.length === i + 1;
    var isUndef = typeof parent[part] === 'undefined';

    if (isLast) {
      if (isUndef) {
        parent[part] = fn();
      }
      else {
        throw new Error(name + ' is already exist');
      }
    }
    else {
      if (isUndef) {
        parent[part] = {};
      }
      parent = parent[part];
    }
  });
};

model.base.js

MyApp.define('Model.Base', function() {
  return Backbone.Model.extend({
    ...
  });
});

model.item.js

MyApp.define('Model.Item', function() {
  return MyApp.Model.Base.extend({
    ...
  });
});