"Too much recursion" error when using intanceof over typeof?

Question

I was making a function to convert all dates in an object to strings and when I used the following function I got a error in FF "too much recursion". (It also fails in IE and chrome)

    function datesToString(obj) {
        if (obj instanceof Object) {
            if (obj instanceof Date) {
                obj = "this part does not matter";
            } else {
                for (var key in obj) { obj[key] = datesToString(obj[key]); }
            }
        }
        return obj;
    }

but when I changed it to use typeof it worked fine?

    function datesToString(obj) {
        if (obj  && typeof obj == "object") {
            if (obj instanceof Date) {
                obj = "this part does not matter";
            } else {
                for (var key in obj) { obj[key] = datesToString(obj[key]); }
            }
        }
        return obj;
    }

It should be the same amount of recursion. Am I missing something? Why am I getting an error with the first example but not the second?

Update: here is an example of the object I was using (in json form)

Contact = {
    "LContactID": "00000000-0000-0000-0000-000000000000",
    "CellPhone": "",
    "HomePhone": "4564564560",
    "OtherPhone": "",
    "BirthDate": new Date(-62135575200000),
    "SSN": "456456456",
    "HowToContact": 1,
    "ContactType": 3,
    "Address1": "123 test",
    "Address2": "apt. 1",
    "City": "Valparaiso",
    "State": "IN",
    "Zip": "46383",
    "FullAddress": "123 test, apt. 1",
    "BuilderID": "",
    "FullAddressWithCityState": "123 test\r\napt. 1\r\nValparaiso, IN 46383",
    "WorkHours": "9-5",
    "ContactTime": "",
    "ContactMethod": "???",
    "IsMilitaryOrSelfEmployed": false,
    "AlternateContactName": "",
    "AlternateContactPhone": "",
    "ContactID": "00000000-0000-0000-0000-000000000000",
    "Password": null,
    "FirstName": "updatetest",
    "MiddleName": null,
    "LastName": "test_",
    "Suffix": null,
    "EmailAddress": null,
    "EmailAddressAlternate": null,
    "WorkPhone": "6546546540",
    "WorkPhoneExt": null,
    "CreatedOn": new Date(1263597309000),
    "CreatedOnOverride": new Date(-62135575200000),
    "ModifiedOn": new Date(1264014749000),
    "SCRep": "",
    "HBCRep": "",
    "PPPRep": "",
    "DPSRep": "",
    "DRSRep": "",
    "RDRep": "",
    "OwnerID": "00000000-0000-0000-0000-000000000000",
    "FullName": "updatetest test_",
    "ImportID": 0,
    "IncludeEmail": false,
    "PreferredLanguage": 1,
    "CarbonCopyList": [],
    "HearAboutUs": 5,
    "HearAboutUsOther": "",
    "init": function() { },
    "update": function() { },
    "load": function() { }
}

It looks like when I remove the method parameters (init,update and load) it works with the instanceof example.

UPDATE: This is a ASP.Net ScriptManager problem it turns out. Sorry for not including the fact that I was working with an ASP.Net website.The ASP.Net ScriptManager prototypes methods to Function which causes an infinity loop when doing a recursive for in loop on a function.

Answer 1

(First, let me say that I tried your code on FF 3.5.7 and it didn't run into an infinite recursion. maybe you used a different input example?)

Anyway, to answer your questions:

(a) Why do you get an infinite recursion? Because an object may refer to itself. If o.f==o then calling your function on o will fire a subsequent call on o.f which is actually o and so on...

(b) Why the difference between the two versions? obj instanceof Object checks if obj is an instance of Object or an instance of a subclass thereof. Given that every Javascript object inherits from Object then this condition is trivially true, and is thus meaningless. Net result is that the first if always succeeds. On the other hand typeof obj == "object" checks that obj is an instance of Object. This may be false, for example, if obj is an instance of String (at which case typeof obj yields String). Hence, the difference.