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.
JavaScript
No Access-Control-Allow-Origin: What CORS Is Really Doing
CORS errors are a browser protecting you from a response it already received. Understanding that one fact makes every CORS error obvious.
Node.js
Finding a Node.js Memory Leak With Heap Snapshots
Memory climbs, the process gets killed, and restarting fixes it for six hours. Here is the diff based method that finds the retainer.
React
Why Your useEffect Runs Twice and Why That Is Correct
React Strict Mode deliberately double invokes your effects in development. It is finding a real bug in your code, not creating one.
Node.js
UnhandledPromiseRejection Crashes Your Node Process Now
It used to be a warning. Since Node 15 it terminates the process, and that change surfaced a lot of latent bugs.
JavaScript
Cannot Read Properties of Undefined: Finding the Culprit
The error names the property, not the reason. Here is how to work backwards from the message to the function that returned nothing.