The event loop, callback hell, promise chaining, async/await, Promise.all, cancellation with AbortController and debugging tips.
JavaScript
Difference Between var, let and const in JavaScript Explained
Scope, hoisting, the temporal dead zone, the classic loop bug, and what const actually protects — with a self-test at the end.
How to Fix CORS Errors in JavaScript Fetch API Requests
Why CORS errors happen, what preflight requests are, and how to fix them properly on the server instead of disabling browser security.