Question

I think the difference has clicked in my head, but I’d just like to be sure.

On the Douglas Crockford page Prototypal Inheritance in JavaScript, he says

In a prototypal system, objects inherit from objects. JavaScript,
however, lacks an operator that performs that operation. Instead it
has a new operator, such that new f() produces a new object that
inherits from f.prototype.

I didn’t really understand what he was trying to say in that sentence so I performed some tests. It seems to me that the key difference is that if I create an object based on another object in a pure prototypal system, then all the parent parent members should be on the prototype of the new object, not on the new object itself.

Here’s the test:

var Person = function(name, age) {
        this.name = name;
        this.age = age;
}
Person.prototype.toString = function(){return this.name + ', ' + this.age};

// The old way...
var jim = new Person("Jim",13);
for (n in jim) {
    if (jim.hasOwnProperty(n)) {
        console.log(n);
     }
}
// This will output 'name' and 'age'.

// The pure way...
var tim = Object.create(new Person("Tim",14));
for (n in tim) {
    if (tim.hasOwnProperty(n)) {
        console.log(n);
     }
}
// This will output nothing because all the members belong to the prototype.
// If I remove the hasOwnProperty check then 'name' and 'age' will be output.

Is my understanding correct that the difference only becomes apparent when testing for members on the object itself?

Answer 1

Your assumptions are correct, but there is another pattern that Douglas doesn’t talk much about – the prototype can be used for properties as well. Your person class could have been written as:

var Person = function(name, age) {
    this.name = name;
    this.age = age;
}
Person.prototype.name = null; //default value if you don't init in ctor
Person.prototype.age = null;
Person.prototype.gender = "male";
Person.prototype.toString = function(){return this.name + ', ' + this.age;};

In this case, iterating over properties of an instance of this class, as you do in your example, would generate no output for the ‘gender’ property.

EDIT 1:
The assignment of name and age in the constructor do make the properties visible by hasOwnProperty (thanks @matt for reminding me of this). The unassigned gender property would not be visible until someone sets it on the instance.

EDIT 2:
To further add to this, I present an alternative inheritance pattern – one that I have personally used for very large projects:

var inherits = function(childCtor, parentCtor) {
  function tempCtor() {};
  tempCtor.prototype = parentCtor.prototype;
  childCtor.superclass = parentCtor.prototype; 
  childCtor.prototype = new tempCtor();
  childCtor.prototype.constructor = childCtor;
};

var Person = function(name){
    this.name = name;
}
Person.prototype.name = "";
Person.prototype.toString = function(){
    return "My name is " + this.name;
}

var OldPerson = function(name, age){
    OldPerson.superclass.constructor.call(this);
    this.age = age
};
inherits(OldPerson, Person);
OldPerson.prototype.age = 0;
OldPerson.prototype.toString = function(){
    var oldString =  OldPerson.superclass.toString.call(this);
    return oldString + " and my age is " + this.age;
}

This is a fairly common pattern with a small twist – the parent class is attached to the child via the “superclass” property permitting you to access methods/properties overridden by the child. Technically, you could replace OldPerson.superclass with Person, however that is not ideal. If you ever changed OldPerson to inherit from a class other than Person, you would have to update all references to Person as well.

EDIT 3:
Just to bring this full circle, here is a version of the “inherits” function which takes advantage of Object.create and functions exactly the same as I previously described:

var inherits = function(childCtor, parentCtor) {
    childCtor.prototype = Object.create(parentCtor.prototype);
    childCtor.superclass = parentCtor.prototype; 
};