systemjs/lib/proto.js

// SystemJS Loader Class and Extension helpers

function SystemJSLoader() {
  SystemLoader.call(this);

  systemJSConstructor.call(this);
}

// inline Object.create-style class extension
function SystemProto() {};
SystemProto.prototype = SystemLoader.prototype;
SystemJSLoader.prototype = new SystemProto();

var systemJSConstructor;

function hook(name, hook) {
  SystemJSLoader.prototype[name] = hook(SystemJSLoader.prototype[name]);
}
function hookConstructor(hook) {
  systemJSConstructor = hook(systemJSConstructor || function() {});
}

function dedupe(deps) {
  var newDeps = [];
  for (var i = 0, l = deps.length; i < l; i++)
    if (indexOf.call(newDeps, deps[i]) == -1)
      newDeps.push(deps[i])
  return newDeps;
}

function extend(a, b, underwrite) {
  for (var p in b) {
    if (!underwrite || !(p in a))
      a[p] = b[p];
  }
}