Understanding behavior of prototype - JavaScript

Question

I don't understand why the following is happening:

I have the following code:

singleton = (function() {
    somePrivFunction = function() {
        return new B();
    }

    A = function() {};
    B = function() {};
    C = function() {};

    A.prototype.a = function() { console.log("a"); };

    B.prototype = A.prototype;
    B.constructor = B;
    B.prototype.b = function() { console.log("b"); };

    C.prototype = A.prototype;
    C.constructor = C;
    C.prototype.c = function() { console.log("c"); };

    return { 
        someFunction: function() { 
            return somePrivFunction(); 
        }
    }
})();

When I call singleton.someFunction() it returns me an instance of B. However all of the following work:

singleton.someFunction().b(); // Prints "b" 
singleton.someFunction().a(); // Prints "a" 
singleton.someFunction().c(); // Prints "c", but why? Shouldn't it be undefined?

You got the A, B,C inheritance all wrong. A Dog is an Animal but an Animal is not necessarily a Dog so setting one prototype to equal another is wrong. You can use Object.create instead. The following answer may help: stackoverflow.com/questions/16063394/… — HMR
– HMR, Commented Nov 17, 2014 at 16:11

William Barbosa · Accepted Answer · 2014-11-17 11:07:58Z

3

B.prototype has a reference to the same object as A.prototype (line 12). On line 16 you stored a reference of this same object to C.prototype. This means all three prototypes point to the same object, therefore all changes made to A.prototype, B.prototype or C.prototype will actually change the same object. At the end of your code, this object has three methods: a, b and c.

answered Nov 17, 2014 at 11:07

William Barbosa

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5 Comments

jaibatrik Over a year ago

Okay, I want B and C to inherit only A's methods, but not each others', is there a way to achieve that?

Dmitry Over a year ago

@jaibatrik Do B.prototype = new A();

HMR Over a year ago

@Dmitry even though creating an instance of Parent to use as prototype of Child accidentally works here without problems I would not advice doing so, it's better to use Object.create

Dmitry Over a year ago

@HMR it is not accidentally works here. It is the only option for older browsers (IE8-). Besides Object.create with only 1 argument is equivalent to new.

HMR Over a year ago

@Dmitry I say accidentally works without problems. Try that again with mutable instance specific members on the Parent. Or certain constructor arguments that are not available when defining Child. So I'll repeat it again: it's better to use Object.create or it's polyfil (like inherits in google closure compiler that has existed for many many years)

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