title: What is a circular structure in JSON?
body: <p>I'm getting an error in my Express app which states:</p>
  
  <blockquote>
    <p>UnhandledPromiseRejectionWarning: TypeError: Converting circular structure to JSON</p>
  </blockquote>
  
  <p>I've searched and found references to similar issues, and maybe I'm being a bit slow but I don't even understand what a "circular strcuture" is in JSON.  I can't seem to find any relevant information online either as far as explaining what exactly it is and why these types of errors occur. How can a list of photo metadata that I'm retrieving from a third party API contain circular references to itself, JSON is ultimately just a string, how can it reference itself?</p>