 <p>Your only option in JSON for a null value is <code>null</code>. <code>""</code> isn't null, it's an empty string. <code>{}</code> is an empty object. <code>[]</code> is an empty array. I would use those where it was appropriate to have empty strings, objects, or arrays, but for null I'd use <code>null</code>.</p> <p>I don't like just leaving the property off entirely (which is what I think you mean by #3), because I prefer that the JSON structure be clear even when a value is omitted. But that's a subjective response, and people can have different opinions about it.</p>