What are the steps to follow to debug memory leak in Jest?

Question

My tests in Jest seem to be leaking :

What are the recommanded steps to debug that kind of issue?

I'm new to that kind of problem. As you can see, I have tried passing in the options documented in Jest (--forceExit --detectOpenHandles --runInBand --logHeapUsage), but that does not solve my problem.

Answer 1

I ran into that I needed to debug some tests leaking memory and found that taking a Heapdump of the process running the tests was trickier than expected. If you add the --inspect flag the cli command it will only inspect the jest process.

In order to attach a debugger to your test executer go into node_modules/jest_worker/build/worker.js and add the --inspect flag to the worker. Jest usually strip the --inspect flag.

Use this code:

_initialize() {
  const args = process.execArgv.filter(v => !/^--(debug|inspect)/.test(v));
  args.push('--inspect');
  const child = (_child_process || _load_child_process()).default.fork(
    require.resolve('./child'),
    // $FlowFixMe: Flow does not work well with Object.assign.
    Object.assign(
      {
        cwd: process.cwd(),
        env: Object.assign({}, process.env, {
          JEST_WORKER_ID: this._options.workerId,
        }),
        // Suppress --debug / --inspect flags while preserving others (like --harmony).
        execArgv: args,
        silent: true,
      },
      this._options.forkOptions,
    ),
  );
  ...
}

Finally run your tests like this: node --expose-gc scripts/test.js --env=jsdom --runInBand --logHeapUsage If you have not ejected your react-scripts you may have to do that first, the change can be reverted after you're done debugging. --runInBand is important else you will get too many workers, jest creates one less worker than you have cores in your computer, so it will be hard to inspect / work with them.

Do not add the --inspect flag to the node --expose-gc scripts/test.js --env=jsdom --runInBand --logHeapUsage command since then the jest process will take the default debugger port.

After this step you run your tests and open Google Chrome, navigate to chrome://inspect/ your process will appear in the list of Remote Targets and from there you can do memory profiling and take heap dumps of your test process.