Jest Worker Ran Out of Memory in CI

By Kaveh Alemi·Latchkey

Jest runs tests across worker processes; when total worker memory exceeds the runner RAM, a worker is OOM-killed and Jest reports it failed unexpectedly. High --maxWorkers is the usual driver.

What this error means

Tests fail with Jest worker encountered 4 child process exceptions or ran out of memory. Lowering --maxWorkers or using a larger runner clears a one-off spike; steadily climbing memory across files suggests a leak.

shell

Jest worker encountered 4 child process exceptions, exceeding retry limit
FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory

Common causes

Too many workers for the runner RAM

Jest defaults to many workers; each holds its own heap, and the sum can exceed available memory.

A memory leak in the test setup

Unclosed handles or accumulating module state grow each worker until it is killed; that is a code fix.

How to fix it

Lower workers and cap the heap

Reduce concurrency so total worker memory fits.

shell

node --max-old-space-size=2048 node_modules/.bin/jest --maxWorkers=2

Right-size or fix the leak

Use a larger runner for genuinely heavy suites.
Run with --detectLeaks / --logHeapUsage to find growth.
Close DB/server handles in afterAll to stop accumulation.

How to prevent it

Set --maxWorkers to the runner size.
Monitor heap usage per worker in CI.
Right-size memory for large test suites.

Frequently asked questions

What causes "Jest worker OOM"?