Edit in JSFiddle

function SuperType(name) {
  this.name = name;
  this.friends = ["kim", "lee", "park"];
}

SuperType.prototype.sayName = function() {
  document.write(this.name + "<br>");
};

function SubType(name, age) {
  SuperType.call(this, name); // SuperType() 두번째 호출
  this.age = age;
}

SubType.prototype = new SuperType() // SuperType() 첫번째 호출
SubType.prototype.constructor = SubType;
SubType.prototype.sayAge = function() {
  document.write(this.age + "<br>");
}