Simple object inheritance in JavaScript

Object.inherit.js

(function () {
    "use strict";

    var copyOwnProperties = function (from, to) {
        for (var propertyName in from) {
            if (from.hasOwnProperty(propertyName)) {
                to[propertyName] = from[propertyName];
            }
        }
    };

    var inherit = function (additionalProperties) {
        var subclass = Object.create(this);

        subclass.create = function () {
            // When calling `MyClass.create()` directly, `this` will be `MyClass` as expected.
            // When passing `MyClass.create` to another function we lose the intended `this` (it's probably set to `window` or null)
            // so explicitly use the `subclass` in that case.
            var prototype = (this && this !== window) ? this : subclass;
            var instance = Object.create(prototype);
            
            // The instance may have an `initialize` method.
            if (typeof instance.initialize === "function") {
                instance.initialize.apply(instance, arguments);
            }
            
            return instance;
        };

        copyOwnProperties(additionalProperties, subclass);

        return subclass;
    };

    Object.inherit = inherit;
} ());

usage.js

var User = Object.inherit({
    initialize: function(name) {
        this.name = name;
    },
    sayHello: function() {
        console.log("Hello, " + this.name);
    }
});


var john = User.create("John");
john.sayHello();

// Can inherit from prototype objects
var SuperUser = User.inherit({
    sayHello: function() {
        console.log("Hello super, " + this.name);
    }
});


// Can pass `create` to other functions and it works as expected
var users = [ "Andrew", "Bob", "Chris" ].map(SuperUser.create);

// users === [ SuperUser.create("Andrew"), SuperUser.create("Bob"), SuperUser.create("Chris") ];

users.forEach(function(user) {
    user.sayHello();
});

Yes, you can't use new or instanceof.

You can use User.isPrototypeOf(john) though.

It makes sense really, because one "object" cannot be an instance of another "object". I suppose instanceof is really asking wasConstructedBy.

I was very confused when I saw this code in the "Programming in HTML5 with JavaScript and CSS3" Microsoft course. Your example really cleared it up for me.