closure.js

var C = function(opts) {
    opts = opts || {};

    function foo() {
        return opts.foo;
    }

    return {
        foo: foo,
    };
}

var c = C({foo: "bar"});
c.foo();

prototype.js

var C = (function() {
    function C(opts) {
        this.opts = opts || {};
    }

    C.prototype.foo = function() {
        return this.opts.foo;
    };

    return C;   
}());

var c = new C({foo: "bar"});
c.foo();