Question

I’m working on a Firefox extension and I’m trying to stringify a JSON object.

I’m using this stringify function but I’m getting this error:

Could not convert JavaScript argument "NS_ERROR_XPC_BAD_CONVERT_JS"

I really just care about the first level or two or properties inside the object, and I don’t care about the methods / functions. Is there a simpler way to stringify an object if I don’t need all this?

Here’s the bit of code I’m using:

    var s=JSONstring.make('abc');

    try{

        Firebug.Console.log(gContextMenu);

        s = JSON.stringify(gContextMenu);

        Firebug.Console.log(s);
    }catch(e){
        Firebug.Console.log('error');
        Firebug.Console.log(e);
    }
    var s=JSONstring.make('abc');
    Firebug.Console.log(s);
    Firebug.Console.log(gContextMenu);

Here is the error in the console window:

This is what I was able to copy out of the Firebug console window:

http://pastebin.com/KPXceRag

Here is a screenshot of the object:

image http://img143.imageshack.us/img143/2603/pictureos.png

Answer 1

You can define a custom function on your object called toJSON() that returns only the elements of the object you want. Somewhat counter-intuitively, your toJSON function should not return a JSON string – it just returns an object representing what should be used as the input for JSON.stringify.

For example:

// an object with some attributes you want and some you don't
var o = {
    a:"value1", 
    b:"value2", 
    doCalc: function() { return this.a + " " +  this.b } 
};
// define a custom toJSON() method
o.toJSON = function() { 
    return {
        a:this.a, 
        calc: this.doCalc() 
    } 
};

JSON.stringify(o); // '{"a":"value","calc":"value1 value2"}'

In your case, you should be able to define this method for gContextMenu on the fly, before calling JSON.stringify. This does require you to explicitly define what you want and don’t want, but I don’t think there’s a better way.

Edit: If you want to pull all non-method values, you could try something like this:

o.toJSON = function() { 
    var attrs = {};
    for (var attr in this) {
        if (typeof this[attr] != "function") {
            attrs[attr] = String(this[attr]); // force to string
        }
    }
    return attrs;
};

You will probably end up with a few attributes set to "[object Object]", but if all you want is inspection, this probably isn’t an issue. You could also try checking for typeof this[attr] == "string" || typeof this[attr] == "number" if you wanted to get only those types of attributes.