Six short snippets with setTimeout, promises, await and queueMicrotask. Predict the order, run it, then read why. The one rule: sync code first, then all microtasks, then the next task.
Send many requests, keep the successes, and report the failures instead of losing everything to one rejected promise. Plus limiting how many run at once.
fetch has no timeout option and can wait a very long time. AbortSignal.timeout(ms) fixes that in one line, and AbortSignal.any combines it with a cancel button.
Read a CSV too big for memory line by line with readline, total a column, and write a filtered copy with pipeline. Memory stays flat no matter how big the file is.
Trace a detached preview from its DOM node to an application Map, repair cleanup, and verify repeated open-and-close cycles with heap snapshots.
Record one slow click in Chrome's Performance panel, read the Bottom-up tab by self time, fix the function it points at, and record again to prove the fix worked.