Javascript inheritance implementation question

Question

In his sitepoint article about javascript inheritance, Harry Fuecks explains a way of implementing inheritance as follows:

    function copyPrototype(descendant, parent) {
    var sConstructor = parent.toString();
    var aMatch = sConstructor.match( /\s*function (.*)\(/ );
    if ( aMatch != null ) { descendant.prototype[aMatch[1]] = parent; }
    for (var m in parent.prototype) {
        descendant.prototype[m] = parent.prototype[m];
    }
};

While I understand his code, one question comes to mind - why not remove the for loop and simply do this:

 function copyPrototype(descendant, parent) {
    var sConstructor = parent.toString();
    var aMatch = sConstructor.match( /\s*function (.*)\(/ );
    if ( aMatch != null ) { descendant.prototype[aMatch[1]] = parent; }
    descendant.prototype = parent.prototype;
};

Thanks.

That's a terrible way to implement inheritance - function decompilation, manual copying of properties (without even taking care of DontEnum bug) instead of more efficient prototypical approach, etc. — kangax
– kangax, Commented Oct 2, 2009 at 2:46
I would suggest to read Crockford's article (crockford.com/javascript/inheritance.html) for a simple beginner-oriented overview of inheritance in JS. For a deeper understanding, look into Michaux's (peter.michaux.ca/articles/class-based-inheritance-in-javascript) or Giammarchi (3site.eu/doc) writings. — kangax
– kangax, Commented Oct 2, 2009 at 5:09
Check this out: stackoverflow.com/questions/7486825/javascript-inheritance/… — Lorenzo Polidori
– Lorenzo Polidori, Commented Oct 10, 2012 at 10:36

Alex Barrett · Accepted Answer · 2009-10-02 01:14:09Z

3

Assigning the prototype of one function to another would only assign a reference to the original prototype; both would be sharing the same prototype object. Iterating through the prototype creates a shallow copy of all its members.

answered Oct 2, 2009 at 1:14

Alex Barrett

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6 Comments

jd. Over a year ago

Wouldn't that be what is wanted when inheriting from another object? Shouldn't any changes to the parent be reflected in the descendants?

Alex Barrett Over a year ago

Under some circumstances, yes. But you certainly don't want changes to the descendant reflected in the parent.

Matthew Lock Over a year ago

Instead of assigning the prototype of one function to another I assign the prototype of my "subclass" function to a new MyParentClass() as outlined on page 169 of the Javascript Rhino book. Is that wrong/bad? PositionedRectangle.prototype = new Rectangle();

Alex Barrett Over a year ago

@Matthew Lock: This is clever technique making use of the prototype model. When using the new operator on a function, an Object is created and the members of the constructor's prototype are copied to that object. By using this new (entirely separate) object as our descendent's prototype, we have effectively created a copy of the original prototype.

jd. Over a year ago

Does that mean that the for loop in the question above can just be replaced by descendant.prototype = new parent(); ?

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