The other day, I ran into a very strange problem with JSON.stringify[^1].

Let's say I have a fairly nested JS object like this and I need to JSON-encode it:

var foo = { 
    "totA": -1,
    "totB": -1,
    "totC": "13,052.00",
    "totHours": 154,
    "groups": [
        {"id": 1,
        "name": "Name A",
        "billingCodes": [
            {"bc": "25", "type": "hours", "hours": "5", "amount": "$25.00"}
        ]}
    ] 
};

If I JSON-encode it using the native browser JSON.stringify (tested in Chrome, Firefox, IE9/10), I get back a JSON string that looks like this (which is what I expect):

Native JSON.stringify JSFiddle example

{
    "totA": -1,
    "totB": -1,
    "totC": "13,052.00",
    "totHours": 154,
    "groups": [
        {
            "id": 1,
            "name": "Name A",
            "billingCodes": [
                {
                    "bc": "25",
                    "type": "hours",
                    "hours": "5",
                    "amount": "$25.00"
                }
            ]
        }
    ]
}

The weirdness comes in if I try to do the same thing on a page that's using either PrototypeJS <= 1.6.

In that case, JSON.stringify on the same object gives me back the following JSON:

ProtypeJS JSON.stringify JSFiddle example

{
    "totA": -1,
    "totB": -1,
    "totC": "13,052.00",
    "totHours": 154,
    "groups": "[{\"id\": 1, \"name\": \"Name A\", \"billingCodes\": [{\"bc\": \"25\", \"type\": \"hours\", \"hours\": \"5\", \"amount\": \"$25.00\"}]}]"
}

Obviously, the above is a problem because it doesn't JSON-decode to the same object that was originally passed to JSON.stringify.

After asking the fine people on StackOverflow I found out several interesting things.

The idea of .toJSON() methods

Both json2.js and native JSON.stringify calls check to see if the object being encoded already has a .toJSON() method and uses that method instead of encoding the object according to the JSON spec.

This was an issue specific to PrototypeJS <= 1.6

There's a bug in one of the PrototypeJS .toJSON() functions that crops up with objects nested inside arrays. PrototypeJS 1.7 has since fixed this issue.

The easiest workaround is just to whack the PrototypeJS .toJSON() functions^2

if(window.Prototype) {
    delete Object.prototype.toJSON;
    delete Array.prototype.toJSON;
    delete Hash.prototype.toJSON;
    delete String.prototype.toJSON;
}

[^1]: Taken from the following StackOverflow questions. https://stackoverflow.com/questions/17618791/noticing-an-odd-difference-between-different-implementations-of-json-stringify and https://stackoverflow.com/questions/710586/json-stringify-bizarreness