Erik Arvidsson warns against extending the JavaScript Object with Object.prototype, as it can lead to breakages in third party code when they use the for(key in obj) method of iterating through any class (since they all are subclasses of Object). Instead Erik recommends extending the classes that directly benefit from the extending functions, and treat Object as a final constant. Erik also suggests that hashes and associative arrays should be done in JavaScript using Object, not Arrays.
Although JavaScript is a class-free, object-oriented language using prototypal inheritance, it can still be used in a classical object-oriented way. Douglas Crockford compares the two inheritance-based systems, adding syntactic sugar to JavaScript to allow classical inheritance, and demonstrates a number of patterns that are not available in classical languages.