Why does `JSON.stringify` serialize arrays as strings when prototype.js is loaded?

The function JSON.stringify() defined in ECMAScript 5 and above (Page 201 - the JSON Object, pseudo-code Page 205), uses the function toJSON() when available on objects.

Because Prototype.js (or another library that you are using) defines an Array.prototype.toJSON() function, arrays are first converted to strings using Array.prototype.toJSON() then string quoted by JSON.stringify(), hence the incorrect extra quotes around the arrays.

The solution is therefore straight-forward and trivial (this is a simplified version of Raphael Schweikert's answer):

delete Array.prototype.toJSON

This produces of course side effects on libraries that rely on a toJSON() function property for arrays. But I find this a minor inconvenience considering the incompatibility with ECMAScript 5.

It must be noted that the JSON Object defined in ECMAScript 5 is efficiently implemented in modern browsers and therefore the best solution is to conform to the standard and modify existing libraries.

As people have pointed out, this is due to Prototype.js - specifically versions prior to 1.7. I had a similar situation but had to have code that operated whether Prototype.js was there or not; this means I can't just delete the Array.prototype.toJSON as I'm not sure what relies on it. For that situation this is the best solution I came up with:

function safeToJSON(item){ 
    if ([1,2,3] === JSON.parse(JSON.stringify([1,2,3]))){
        return JSON.stringify(item); //sane behavior
    } else { 
        return item.toJSON(); // Prototype.js nonsense
    }
}

Hopefully it will help someone.