"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", "Oth…

Accepted Answer

(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.

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

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