扩展和覆盖JS类的私有和公共方法

Question

In ES5 , having the definition of a base class that provides a public API:

var Class = function(name) {
    var self = this;
    var name = name;

    return {
        print: function() {
          printDetails();
          return this;
        },
        iAmA: function() {
            console.log('I am an instance of:', self.constructor.name);
        }
    };

    function printDetails() {
        console.log('Name', name);
    };
};

How can I:

• extend it, without instantiate it

  Is Child.prototype = Class without new wrong? I just want to define the classes and the inheritance and not instantiate any class yet.

• override a private method (and make it callable through public method)

  I want Child.print() to call Child.printDetails() , instead of Class.printDetails() .

• inherit the Class public api without redeclaring it

  I can return new Class(name) from Child , but I won't get my new Child methods to be called ( Class methods are called instead of the overridden ones)

• add a method to the api without redeclaring it all

  I don't know if it is possible, but my goal is to inherit as much as possible and define just the additions/variations.

The following is what I came up with, but still I don't like to instantiate a new Class just to get the public interface. I would like to specify just eventual new public methods.

var Child = function(name, age) {
    var adult = age > 21 ? true : false;

    //return new Class(name);
    var api = new Class(name);
    api.print = function() {
        printDetails();
        return this;
    }
    return api;

    function printDetails() {
        console.log('Child name', name, 'Adult', adult ? 'Yes' : 'No');
    }
};

Child.prototype = Class;
Child.prototype.constructor = Child;

Child.prototype.printDetails = function() {
    console.log('Child name', name, 'Adult', adult ? 'Yes' : 'No');
}

var cls = new Class('Class');
var chd = new Child('Child', 22);

cls.print();
cls.iAmA();
chd.print();
chd.iAmA();

Answer 1

In ES5, having the definition of a base class

That's not what we would usually call a class - which would involve instantiation via new and prototype inheritance. It's a function returning an object, a factory function .

We can still use parasitical inheritance to create new objects with an extended API however.

How can I extend it, without instantiate it? How can I inherit the Class public api without redeclaring it?

Just declare a new function that returns the same kind of object:

function ChildClass(name) {
    return Class(name);
}

Is Child.prototype = Class without new wrong?

Yes. But it would be wrong with new either - we just don't need the prototype here.

How can I override a private method (and make it callable through public method)

You cannot. Private things stay private to the class they are declared in, they cannot be overridden.

I want Child.print() to call Child.printDetails() , instead of Class.printDetails() .

You can however overwrite the public print method to do something different. You successfully did that in your attempt.

How can I add a method to the api without redeclaring it all

Add it to the object you return, no need to copy everything to a new object ("redeclaring"):

function ChildClass(name, age) {
    var obj = Class(name);
    obj.newMethod = function() { return age; }
    return obj;
}

The following is what I came up with

Yes, your Child function is exactly what it should look like (maybe except for the new in front of Class which you don't need).

However, you should drop all that prototype stuff (the three assignments) from your code. Your constructor does not use this anywhere and returns an object, so it's ignored anyway.