title: What does it mean when an HTTP request returns status code 0?
body: <p>What does it mean when JavaScript network calls such as fetch or XMLHttpRequest, or any other type of HTTP network request, fail with an HTTP status code of 0?</p>
  
  <p>This doesn't seem to be a valid HTTP status code as other codes are three digits in HTTP specification. </p>
  
  <p>I tried unplugging the network completely as a test. It may be unrelated, but that resulted in status code 17003 (IIRC), which cursory searching suggests means "DNS server lookup failed".</p>
  
  <p>The same code works fine from some locations and systems, however within certain environments it fails with status code 0 and there is no responseText provided.</p>
  
  <p>This is a typical HTTP POST to an Internet URL. It does not involve file:// which I understand may return 0 indicating success in Firefox. </p>