Javascript Subclassing. Setting the constructor property

Question

For years I've been writing/extending my classes like so:

function Container(name, contents, logErrors){
    this.name = name;
    this.contents = contents;
    this.logErrors = logErrors || false;
}

function Group(contents){
    this.id = new Date().getTime();
    Container.call(this, 'Group_'+ this.id, contents);
}
Group.prototype = Object.create(Container.prototype);
Group.constructor = Group; 

and yet, in some places I've seen the constructor property being assigned on the prototype of the subclass rather than directly on the subclass:

function Group(contents){
    this.id = new Date().getTime();
    Container.call(this, 'Group_'+ this.id, contents);
}
Group.prototype = Object.create(Container.prototype);
Group.prototype.constructor = Group; // <-----

Which is correct?

a) Group.prototype.constructor = Group;  
b) Group.constructor = Group;  
c) both a AND b  
d) neither a nor b  

Please cite your source if possible.

Answer 1

You should always use a) Here's the reason why.

function Container(){
    // code
}
function Group(){
    // code
}

Observe that at this point of time

console.log(Group.prototype.constructor === Group);
// true
console.log(Group.constructor === Function);
// true

Now if you do

Group.prototype = Object.create(Container.prototype);

You loose the original Group.prototype and replaces all its methods as well. Which means you also lose the original Group.prototype.constructor as well.

Hence you may observe that at this point of time.

console.log(Group.prototype.constructor === Container);
// true

Now if you want something like a copy method.

Group.prototype.copy = function() {  
    return new this.constructor(this.contents);
};

You may end up getting the result

var group1 = new Group();
console.log(group1.copy() instanceof Group);
// false

Which may not be expected.

But if you would have done

Group.prototype.constructor = Group;

Then the results are as expected

console.log(group1.copy() instanceof Group);
// true

Also you can read more here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object-Oriented_JavaScript#Inheritance

Hope that helps.

Answer 2

Only a) is correct. The point of the .constructor assignment is that instances of your subclass (i.e. Group) inherit a .constructor property that points to the subclass constructor. They do inherit it from the subclasses' prototype object (i.e. Group.prototype) and nothing else.

You can also omit the statement entirely if none of your code is using the .constructor property.