async function criticalFunction() {
  const data = await getDataFromDb();
  const modifiedData = await asynchronouslyDoStuffWithData(data);
  return writeDataBackToDb(modifiedData);
}

const lock = new Semaphore(1);

async function criticalFunctionSynchronous() {
  await lock.acquire();

  try {
    await criticalFunction();
  } finally {
    lock.release();
  }
}

import Semaphore from 'semaphore-async-await';

(async () => {

  // A Semaphore with one permit is a lock
  const lock = new Semaphore(1);

  // Helper function used to wait for the given number of milliseconds
  const wait = (ms) => new Promise(r => setTimeout(r, ms));

  let globalVar = 0;

  (async () => {
    // This waits (without blocking the event loop) until a permit becomes available
    await lock.wait();
    const localCopy = globalVar;
    await wait(500);
    globalVar = localCopy + 1;
    // Signal releases the lock and lets other things run
    lock.signal();
  })();

  // This returns false because the function above has acquired the lock
  // and is scheduled to continue executing once the main function yields or
  // returns
  console.log(lock.tryAcquire() === false);

  // Similar to the function above but using waitFor instead of wait. We
  // give it five seconds to wait which is enough time for it to acquire
  // the lock
  (async () => {
    // This waits for at least five seconds, trying to acquire a permit.
    const didAcquireLock = await lock.waitFor(5000);
    if (didAcquireLock) {
      const localCopy = globalVar;
      await wait(500);
      globalVar = localCopy + 1;
      // Signal releases the lock and lets other things run
      lock.signal();
    }
  })();

  // Alternative to using wait()/signal() directly
  lock.execute(async () => {
    const localCopy = globalVar;
    await wait(500);
    globalVar = localCopy + 1;
  });

  // Wait for everything to finish
  await wait(2000);

  console.log(globalVar === 3);
})();