test.js

(function() {
    document.writeln("HTMLElement.prototype = " + HTMLElement.prototype);
    var something = document.getElementById('something');
    document.writeln("something = " + something);
    document.writeln("something.prototype = " + something.prototype);
    document.writeln("something.__proto__ = " + something.__proto__);

    document.writeln("HTMLElement.prototype === something.__proto__ = " +
                     (HTMLElement.prototype === something.__proto__));

    HTMLElement.prototype.doSomething = function() {
        document.writeln("i'm in " + this + " doing something");
    };

    something.doSomething();
})();