So, that error generally comes from malformed JSON. To track down exactly what is going on you will need to debug at each step and figure out where this is happening. I took a look, but it’s not possible (for me at least) to tell on just reading through it.
I would console.log() your data at each stage that it is being queried or manipulated in some way. Check that the JSON is valid at each step.
Let me know if that is enough to get you started.