Question

I received this error, and I couldn’t find any reasonable answer to this question, so I thought I’d write a summary of the problem.

If you run this snippet in irb:

JSON.parse( nil )

You’ll see the following error:

TypeError: can't convert nil into String

I was kind of expecting the function to return nil, and not a TypeError. If you convert all input using to_s, then you’ll see the octet error:

JSON::ParserError: A JSON text must at least contain two octets!

That’s just fine and well. If you don’t know what an octet is, read this post for a summary and solution:
What is a JSON octet and why are two required?

Solution

The variable you’re passing in is an empty string. Don’t attempt to use an empty string in the JSON.parse method.

Question

So, now I know the cause of the error, what pattern should I use to handle this? I’m a bit loathe to monkey patch the JSON library to allow nil values. Any suggestions would be greatly appreciated.

Answer 1

parsed = json && json.length >= 2 ? JSON.parse(json) : nil

But really the library should be able to handle this case and return nil. Web browsers with built-in JSON support seem to work just like you expect after all.

---

Or to do it with a only slightly intrusive mini patch:

module JSON
  def self.parse_nil(json)
    JSON.parse(json) if json && json.length >= 2
  end
end

parsed = JSON.parse_nil(json)