Move a slow calculation off the event loop with a worker thread

One endpoint that resizes an image is making every other request on the server wait its turn.

Intermediate to advancedRead 6 minDoing: about an hour7 steps

What you'll have at the end

The same calculation running on a worker thread, proven by a health check that keeps answering in a few milliseconds even while the heavy request is still computing, tested against both a single request and a burst of several at once.

Before you start

You need

A running HTTP server with one route that does a synchronous, CPU-heavy calculation, plus a way to send it more than one request at a time so you can watch how it behaves under load.

Not covered

Cutting how much data crosses between the main thread and a worker, by transferring a buffer instead of copying it, is a separate tuning problem this recipe does not cover.

Leans on

Node.js Cheat SheetCovers worker threads, sending messages to one, and the specially cased error event a running thread can raise.
JavaScript Cheat SheetCovers the promise this recipe wraps a worker in, so the route can await its answer like any other async call.

7 steps

Checked 25 Aug 2026

Part of the Node.js cookbook