Jest "JavaScript heap out of memory" in CI (--maxWorkers)

A Jest worker exhausted the V8 heap and crashed. Jest spawns one worker per CPU, so on a many-core CI machine the combined memory blows past the limit; a per-test leak makes it worse.

What this error means

The run dies with "FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory." There is often no assertion failure - a worker was OOM-killed, not a test failing.

<--- Last few GCs --->
[1234:0x5e0] Mark-Compact ... allocation failure

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

Common causes

Too many workers for the RAM

Jest defaults to one worker per core. A 16-vCPU runner spawns ~15 workers; if each loads a heavy module graph, total memory exceeds the limit.

A memory leak across tests

Tests that retain large objects, never tear down servers, or accumulate listeners grow a worker's heap until V8 gives up.

How to fix it

Cap the worker count in CI

Fewer workers means lower peak memory - the most reliable fix on big runners.

# an absolute count or a fraction of cores
jest --maxWorkers=2
# or
jest --maxWorkers=50%

Raise the Node heap and find leaks

NODE_OPTIONS=--max-old-space-size=4096 jest --maxWorkers=2
# investigate retained memory
jest --detectLeaks --logHeapUsage

How to prevent it

• Pin --maxWorkers in CI rather than inheriting the core count.
• Tear down servers, timers, and listeners in afterEach/afterAll.
• Size the runner for the suite's real memory footprint.

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