Low-Hanging Fruits of JS build and test times

Speeding up Jest test suite

The InsightAppSec frontend has about 4000 unit tests written in Jest. Depending on whether or not your laptop's plugged in, they take in or around 6 minutes to run. I recently spent an afternoon doing some benchmarking to try and make them run faster.

Using Node 20 instead of Node 18 made the test suite run 35% faster (5m31s to 3m21s), which is a really nice improvement.

Running tests selectively based on git changes (jest --onlyChanged) has no downsides for regular inner-loop software development (all the tests will still be run on the CI pipeline). But this small change brought the average run time down to about a minute, depending on how many files I've changed.

Speeding up webpack local dev server

Configuring webpack to use caching when running a local dev server took npm run dev from taking 1 minute 40 seconds to run, to taking 12 seconds to run. It works by effectively making npm run dev use HMR from cold.

Leverage existing boring tools

I couldn't believe how much faster these really simple changes made the build and test times. Moving the test suite from Jest to Vitest would probably have taken me a month, and the same goes for swapping webpack with RSpack or Vite (we use a good few plugins).

In the future I'm going to be a bit more wary about big build/test tool migrations for performance reasons because there's still a lot of low-hanging fruit with these slightly older tools. It is well worth doing a bit of profiling before doing a costly tooling migration.

In the meantime I'm enjoying the speedups. I get an hour or so back every week and running tests doesn't take me out of that nice flow state quite like it used to.