Jest Memory Leaks - some lessons learnt

  • 3rd May 2022
  • • 
  • 3 min read
  • Last updated on 18th Nov 2023

Project Infos

  • Nest.JS v8
  • Node.JS v14
  • ~800 unit tests using Jest

The Build Up

Every once in a while, the GitHub Actions pipeline would run out of memory during a test run. This is what it looked like:

<--- JS stacktrace --->

FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory

While it didn't happen consistently, it was still becoming annoying enough. To make matters worse, a Node.JS update to v16 seemed to make the problem unbearable. Time to investigate 🕵🏽‍♂️.

Node v16

There seems to be a bug, introduced with Node v16.11, causing memory leaks with Jest. While this bug is certainly putting my Node 16 update to a halt until it's resolved, the mentioned Github issue also gives quite some insight on how to debug these kinds of issues. Back to Node 14 🔙.

Early Suspicions

Some early ideas about what could be causing the issues were:

  • A bad jest/ts-jest/... package update
  • Jest-ts-auto-mock as this is messing with the transformer
  • The setupFilesBeforeEnv which I had added recently

In order to verify any assumptions and changes made, the following command was used to see if any memory was leaking:

node --expose-gc ./node_modules/.bin/jest --runInBand --logHeapUsage

Going Back in Time

In order to see if the problem has been happening for a while, just at a smaller scale, I went back to a commit some months ago and ran the above command. Turns out, back then everything was looking fine!

With that knowledge, I proceeded to test whether package updates are responsible for what's happening, but no luck there.

I figured I should try and pinpoint the exact commit where the problem started, but with the normal memory fluctuation, it was impossible to spot. To make matters even worse, there seems to be a different memory issue when using the --runInBand flag.

detectLeaks Flag to the Rescue

Thankfully there is yet another flag called --detectLeaks which will force the garbage collection and fail the test suite if it detects any memory leak.

The updated command: node --expose-gc ./node_modules/.bin/jest --detectLeaks

Going back to the same "suspected-memory-leak-free-commit", this command proved exactly that. Jumping around commits "bisection" style, while running the leak detection every time, very quickly gave me the exact commit which turned every test suite red.

setupFilesAfterEnv + nock

Turns out, the issue was the following jest setup file which I introduced to globally disable any external HTTP requests in tests:

// jest.setup.ts
import * as nock from 'nock';


There is a section in the nock README that describes that exact issue. There is also some more context linked there.

Key lessons learnt

  • RTFM
  • Run tests with --detectLeaks flag, perhaps even in the CI to detect issues right away, instead of facing 5 different issues at the same time once it all bubbles up.
  • Try out newer transformers such as swc/jest or esbuild/jest for new projects.