JavaScript is a prototype-oriented language. It can build actual objects from a constructor function and it has almost any feature that any object could have: Constructor. Methods (i.e. functions in JavaScript). Properties (since ECMA-Script 5, "getters/setters"). Instances. In JavaScript, any object has a prototype, including functions. The prototype itself is a rudimentary way of adding object members to any newly created instance of the whole object. var constructor = function() { }; constructor.prototype.text = "hello world"; alert(new constructor().text); // This alerts hello world Why JavaScript isn't an object-oriented programming (scripting) language? Because it has no feature that fits the requirements of the definition of object-oriented programming: Polymorphism: No. You can change the behavior of a prototype member, but this is just reusing the identifier. You aren't able to access the previous implementation of the member in a pseudo-derived object. Inheritance: Not at all. Maybe prototype chain might be comparable to inheritance but JavaScript (ECMA-Script 5.x or earlier versions) has no syntax-based inheritance like other OOP-based languages (i.e. Java, C#, Ruby, Python, VisualBasic.NET, ...). Encapsulation. Yes, of course, but there's no way to create actual private or internal object members. Perhaps I forgot to mention some other detail, but I honestly believe that this is a good summary. Update and summary The core difference is an object-oriented programming language has the features that an object-oriented paradigm must have in order to be considered an object-oriented programming language. Thus, JavaScript, for now, isn't an actual object-oriented programming language because it lacks actual polymorphism and inheritance.