function extend(destClass, srcClass) {
  var destProto = destClass.prototype;
  var srcProto = srcClass.prototype;
  for (var method in srcProto) {
    if (!destProto[method]) {
      destProto[method] = srcProto[method];
    }
  }
}
function Book(){}
Book.prototype.printName = function(){
  console.log('I am a book, named hello');
};
function JS(){}
extend(JS, Book);
var js = new JS();
console.log(js);

function extend(destClass, srcClass, methods) {
  var srcProto = srcClass.prototype;
  var destProto = destClass.prototype ;  
  for (var i=0; i<methods.length; i++) {
    var method = methods[i];
    if (!destProto[method]) {
      destProto[method] = srcProto[method];
    }
  }
}
function Book() {}
Book.prototype.getName = function() {};
Book.prototype.setName = function() {};
function JS() {}
extend(JS, Book, ['getName']);
var js = new JS();
console.log(js);

function extend(destClass) {
  var classes = Array.prototype.slice.call(arguments, 1);
  for (var i=0; i<classes.length; i++) {
    var srcClass = classes[i];
    var srcProto = srcClass.prototype;
    var destProto = destClass.prototype; 
    for (var method in srcProto) {
      if (!destProto[method]) {
        destProto[method] = srcProto[method];
      }
    }
  }
}
function Book() {}
Book.prototype.getName = function() {};
Book.prototype.setName = function() {};
function Tech(){}
Tech.prototype.showTech = function(){};
function JS() {}
extend(JS, Book, Tech);
var js = new JS();
console.log(js);