How do you use JSON.stringify in a custom toJSON method?

Question

So, JSON.stringify provides a great way to turn a JS object like:

var baz = {"foo":1, "bar":someFunction};

in to a JSON string like:

{"foo":1}

It does this with an optional second argument that controls which fields should be serialized:

JSON.stringify(baz, ["foo"]);

That's great, but there's a problem. Let's say your "baz" is actually the property of another object, and you want to serialize that other object:

someObject.baz = {"foo":1, "bar":someFunction};
JSON.stringify(someObject, ["baz"]);

Well, normally you would just define a toJSON method on baz, eg.:

someObject.baz = {"foo":1, "bar":someFunction};
someObject.baz.toJSON = function() { /* logic to "toJSON" baz*/ }
JSON.stringify(someObject, ["baz"]);

Now, as I mentioned earlier, we have the perfect logic to "toJSON" baz already:

someObject.baz.toJSON = function() {
    return JSON.stringify(baz, ["foo"]);
}

but if you try putting that in to your toJSON, you'll get a recursion error, because stringify will trigger the toJSON, which will trigger the stringify, which will ... :-(

You can work around this with a hack:

someObject.baz.toJSON = function() {
    var oldToJON = this.toJSON;
    this.toJSON = null;
    var ret = JSON.stringify(baz, ["foo"]);
    this.toJSON = oldToJON;
    return ret;
}

But ... that just seems wrong. So, my question is: is there any way you can utilize the nifty built-in serialization power of JSON.stringify inside a toJSON method of an object (without having to hide the toJSON method itself during the stringify operation)?

Answer 1

Crockford's json2.js says:

A toJSON method does not serialize: it returns the value represented by the name/value pair that should be serialized, or undefined if nothing should be serialized.

So you are simply expected to return the value that you want serialized. In your case, baz.toJSON should simply return the portion of the baz object that you want serialized:

someObject.baz.toJSON = function() {
  return { foo: this.foo };
};