classical.html

<script>
    var Test = function () {};

    Test.prototype.a = function () {};
    Test.prototype.b = function () {};
    Test.prototype.c = function () {};
    Test.prototype.d = function () {};
    Test.prototype.e = function () {};
    Test.prototype.f = function () {};
    Test.prototype.g = function () {};
    Test.prototype.h = function () {};
    Test.prototype.i = function () {};
    Test.prototype.j = function () {};
    Test.prototype.k = function () {};

    window.ary = [];
    for (var i = 0; i < 10000; ++i) {
       ary.push(new Test());
    }
</script>

module.html

<script>
    var Test = function () {
        var test = {};
        test.a = function () {};
        test.b = function () {};
        test.c = function () {};
        test.d = function () {};
        test.e = function () {};
        test.f = function () {};
        test.g = function () {};
        test.h = function () {};
        test.i = function () {};
        test.j = function () {};
        test.k = function () {};
        return test;
    };

    window.ary = [];
    for (var i = 0; i < 10000; ++i) {
       ary.push(Test());
    }
</script>

These benchmarks compare the memory usage for classical vs. module pattern objects.

In my tests (Chrome 22.0.1229.94), classical.html generates a 1011 KB heap, while module.html generates a 5.3 MB heap. Most (76%) of the module heap is closures -- one for each function for each instance.

Conclusion: beware using the module pattern for heavily allocated classes.

Real-world example from @domenic: jsdom/jsdom#509, fixed by chad3814/CSSStyleDeclaration@33d2ba3.