Inverse of jQuery.extend(true, …)

Question

I'm looking to reduce storage requirements for JSON data by deltifying it against a known set of defaults. Basically, what I want is an inverse for jQuery's .extend() function, such that the following test passes for arbitrary JSON-compatible objects:

function test_delta(defaults, delta) {
    var current = $.extend(true, {}, defaults, delta);

    QUnit.same(get_delta(current, defaults), delta);
}

Before I start writing my own get_delta(), is anyone aware of an existing implementation?