Merge Javascript objects recursively

merge.js

function merge(obj1, obj2) {
    var isAnyType = function(obj) { return (typeof obj !== "undefined" ); }
    var isSingleType = function(obj) { return (typeof obj === "string" || typeof obj === "number"); }
    var isObjectType = function(obj) { return (typeof obj === "object" && !isArrayType(obj)); }
    var isArrayType = function(obj) { return Array.isArray(obj); }

    if (isAnyType(obj1) && !isAnyType(obj2)) {
        return obj1;
    }

    else if (isAnyType(obj2) && !isAnyType(obj1)) {
        return obj2;
    }

    else if (isSingleType(obj1) && isSingleType(obj2)) {
        return obj1;
    }

    else if (isSingleType(obj1) && !isSingleType(obj2)) {
        return obj2;
    }

    else if (isSingleType(obj2) && !isSingleType(obj1)) {
        return obj1;
    }

    else if ((isObjectType(obj1) && isArrayType(obj2)) || 
        (isObjectType(obj2) && isArrayType(obj1))) {
        throw "Illegal Array and Object merge";
    }

    else if (isArrayType(obj1) && isArrayType(obj2)) {
        return obj1.concat(obj2);
    }

    else if (isObjectType(obj1) && isObjectType(obj2)) {
        retObj = {};
        for (var attr in obj1) {
            retObj[attr] = merge(obj1[attr], obj2[attr]);
        }
        for (var attr in obj2) {
            if (obj1.hasOwnProperty(attr)) continue;
            retObj[attr] = merge(obj2[attr], obj1[attr]);
        }
        return retObj;
    }
}