Networking
502 Bad Gateway From Nginx: Which Side Is Actually Broken
The proxy is telling you your application failed. Here is how to read the error log line and identify which of six things went wrong.
Node.js
EADDRINUSE: Finding and Killing Whatever Took Your Port
Address already in use. Here is how to find the process, why it sometimes is not a process at all, and why the port stays busy after you kill it.
Networking
x509: certificate signed by unknown authority
Your browser trusts the certificate and your application does not. They use different trust stores, and inside a container there may not be one at all.
JavaScript
Unexpected token < in JSON at position 0 Means You Got HTML
The angle bracket is the first character of an HTML error page. Your API returned a page, not JSON, and here is how to find out which page.
Networking
Your Cache-Control Header Is Probably Wrong
no-cache does not mean do not cache. max-age on a CDN is not what you think. Here is what each directive actually does.
JavaScript
No Access-Control-Allow-Origin Header Is Present: What CORS Is Actually Doing
CORS errors are a browser protecting you from a response it already received. Understanding that one fact makes every CORS error obvious.