What to do if an Out of Memory error crashes Jest tests?

The out-of-memory error when running unit tests using the Jest framework most frequently occurs in large projects with a large number of test files. By default, Node.js allocates a strictly limited amount of memory for a single process to run, and when executing hundreds of tests in parallel, this limit is quickly exhausted.

To solve this problem, you need to programmatically increase the allocated memory limit for the Node.js runtime environment before starting the test process. This can be done directly in the package.json configuration file in the scripts section.

In Linux and macOS operating systems, an environment variable or a special node flag specifying megabytes, for example, --max-old-space-size=4096, is added before the test command. For cross-platform compatibility, it is recommended to use special packages that correctly handle memory flags on any operating systems.

In addition, you should optimize the tests themselves. Make sure that after running each test, you correctly clear mocks, close database connections, and stop timers. Reducing the degree of parallelism via special command-line switches can also help stabilize the testing process on weaker machines.