递归地扩展和合并javascript对象

Question

我已经编写了两种扩展和合并javascript对象的方法。这可能是个坏问题，但我不确定我做得对。你能看一下代码，告诉我是否需要改进一些东西，如果我做错了什么（代码工作乍一看）

merge: function (a, b) {
    var copy = {};
    CL.extend(copy, a, [HTMLElement, Array]);
    CL.extend(copy, b, [HTMLElement, Array]);
    return copy;
},

extend: function extend(a, b, excludeInstances) {
    for (var prop in b)
        if (b.hasOwnProperty(prop)) {
            var isInstanceOfExcluded = false;
            if (excludeInstances)
                for (var i = 0; i < excludeInstances.length; i++)
                    if (b[prop] instanceof excludeInstances[i])
                        isInstanceOfExcluded = true;

            if (typeof b[prop] === 'object' && !isInstanceOfExcluded) {
                a[prop] = a[prop] !== undefined ? a[prop] : {};
                extend(a[prop], b[prop], excludeInstances);
            } else
                a[prop] = b[prop];
        }
}