Javascript and OOP

OOP is defined by three things: encapsulation, polymorphism and inheritance. Douglas Crockford's article claims it supports all three so it is an object-oriented language.

var AnimalClass = function Animal(name) {
this.name = name;

// private method
function sayPrivate() {
return "sayPrivate";
};

this.sayPrivileged = function() {
return sayPrivate();
}
}

// public method is added to the prototype
AnimalClass.prototype.say = function (something) {
return this.name + something;
}

var anAnimal = new AnimalClass("foo");
alert(anAnimal.name);
alert(anAnimal.say("ha"));
alert(anAnimal.sayPrivileged("ha"));

Typical way to implement inheritance in Javascript is via object-augmentation. For example, the underscore library defines the following function to extend any given object with the properties of the passed in object.
// Extend a given object with all the properties in passed-in object(s).
_.extend = function(obj) {
each(slice.call(arguments, 1), function(source) {
for (var prop in source) {
if (source[prop] !== void 0) obj[prop] = source[prop];
}
});
return obj;
};

_.extend(anAnimal, { "foo" : "bar" });
alert(anAnimal.foo);