Function shouldRunInBand

packages/jest-core/src/testSchedulerHelper.ts:13–56

(
  tests: Array<Test>,
  timings: Array<number>,
  {
    detectOpenHandles,
    maxWorkers,
    runInBand,
    watch,
    watchAll,
    workerIdleMemoryLimit,
  }: Config.GlobalConfig,
)

const SLOW_TEST_TIME = 1000;

export function shouldRunInBand(
  tests: Array<Test>,
  timings: Array<number>,
  {
    detectOpenHandles,
    maxWorkers,
    runInBand,
    watch,
    watchAll,
    workerIdleMemoryLimit,
  }: Config.GlobalConfig,
): boolean {
  // If user asked for run in band, respect that.
  // detectOpenHandles makes no sense without runInBand, because it cannot detect leaks in workers
  if (runInBand || detectOpenHandles) {
    return true;
  }

  /*
   * If we are using watch/watchAll mode, don't schedule anything in the main
   * thread to keep the TTY responsive and to prevent watch mode crashes caused
   * by leaks (improper test teardown).
   */
  if (watch || watchAll) {
    return false;
  }

  /*
   * Otherwise, run in band if we only have one test or one worker available.
   * Also, if we are confident from previous runs that the tests will finish
   * quickly we also run in band to reduce the overhead of spawning workers.
   */
  const areFastTests = timings.every(timing => timing < SLOW_TEST_TIME);
  const oneWorkerOrLess = maxWorkers <= 1;
  const oneTestOrLess = tests.length <= 1;

  return (
    // When specifying a memory limit, workers should be used
    workerIdleMemoryLimit === undefined &&
    (oneWorkerOrLess ||
      oneTestOrLess ||
      (tests.length <= 20 && timings.length > 0 && areFastTests))
  );
}