Javascript经典继承调用父

Question

I want to call the super method in an extended javascript 'class' by applying classical inheritance.

function Person(name, age) {
    this._name = name;
    this._age = age;
}
Person.prototype.exposeInfo = function() {
    alert(this._name + ' - ' + this._age);    
}

function Employee(name, age) {
    this.parent.constructor.call(this, name, age);
}
Employee.prototype.exposeInfo = function() {
    alert('Call employee');
    this.parent.exposeInfo();    
}

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.parent = Person.prototype;


var p1 = new Person('John Doe', 30);
p1.exposeInfo();

var p2 = new Employee('John Foobar (empl.)', 35);
p2.exposeInfo();

JS Fiddle

The problem is that the method is not being called in the extended class, but only in the parent (Person).

Answer 1

That's because the overriding exposeInfo is being attached to the former prototype object, which is then replaced:

Employee.prototype = Object.create(Person.prototype);

You'll want to reverse the order, attaching methods after creating the prototype :

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.parent = Person.prototype;

Employee.prototype.exposeInfo = function() {
    // ...
}

---

You'll also need to use .call() or .apply() with exposeInfo as you did with the constructor:

Employee.prototype.exposeInfo = function() {
    alert('Call employee');
    this.parent.exposeInfo.apply(this, arguments);    
}

Otherwise, the value of this will be determined by the last member operator:

// so, calling:
this.parent.exposeInfo();

// is equivalent to:
alert(this.parent._name + ' - ' + this.parent._age);

Answer 2

// ...

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.parent = Person.prototype;

Employee.prototype.exposeInfo = function() {
  this.parent.exposeInfo.apply(this, arguments);
  // ...
}

It's not going to work.

Example:

// ...

Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.parent = Person.prototype;

Employee.prototype.exposeInfo = function() {
  this.parent.exposeInfo.apply(this, arguments);
  // ...
}

ParttimeEmployee = Object.create(Employee.prototype);
ParttimeEmployee.prototype.constructor = ParttimeEmployee;
ParttimeEmployee.prototype.parent = Employee.prototype;

ParttimeEmployee.prototype.exposeInfo = function() {
  this.parent.exposeInfo.apply(this, arguments);
  // ...
}

var p1 = new Person('Jack', 30);
p1.exposeInfo(); // ok

var p2 = new Employee('Jane', 25);
p2.exposeInfo(); // ok

var p3 = new ParttimeEmployee('John', 20);
p3.exposeInfo(); // infinite recursion !!!

Correct version:

// Person
function Person(name, age) {
  this._name = name;
  this._age = age;
}
Person.prototype.exposeInfo = function() {
  alert(this._name + ' - ' + this._age);    
}

// Employee
Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.parent = Person.prototype; // <--

Employee.prototype.exposeInfo = function() {
  Employee.parent.exposeInfo.apply(this, arguments); // <--
  // ...
}

// ParttimeEmployee
ParttimeEmployee = Object.create(Employee.prototype);
ParttimeEmployee.prototype.constructor = ParttimeEmployee;
ParttimeEmployee.parent = Employee.prototype; // <--

ParttimeEmployee.prototype.exposeInfo = function() {
  ParttimeEmployee.parent.exposeInfo.apply(this, arguments); // <--
  // ...
}

var p1 = new Person('Jack', 30);
p1.exposeInfo(); // ok

var p2 = new Employee('Jane', 25);
p2.exposeInfo(); // ok

var p3 = new ParttimeEmployee('John', 20);
p3.exposeInfo(); // ok