README.md

JavaScript is in no way immune from race conditions.

With JavaScript's most common use case being web application (both front-end and back-end), input and output are therefore not synchronous. Hence the need devise synchronization paradigmes. Transactional integrity, for example, will greatly benefit from any such exercise.

Below is an example usage of a createMutex function that was written in the aim of introducing a primitive similar to mutex locks to JavaScript.

Usage

const { createMutexLock } = require('./mutex');

const acquireLock = createMutexLock();

const someFunctionWithCriticalSection = async () => {
  
  await acquireLock(async () => {
    await foo();
    await bar();
    await baz();
  });
  
};

Promise.all([
  someFunctionWithCriticalSection(),
  someFunctionWithCriticalSection()
])
  .catch(error => { console.error(error); })

Without mutex

const someFunctionWithCriticalSection = async () => {
  
  // These, collectively, represent critical sections
  await foo();
  await bar();
  await baz();
  
};

Promise.all([
  someFunctionWithCriticalSection(),
  someFunctionWithCriticalSection()
])
  .catch(error => { console.error(error); })

mutex.js

/**
 * Create a promise-returning function that accepts another promise-returning
 * function as its parameter. Call it for some transactional integrity,
 * at least at the process level
 *
 * @returns {(param: () => Promise) => Promise}
 */
const createMutexLock = () => {

  let locked = false;

  let listeners = [];

  const runNextListener = () => {
    const listener = listeners.pop();
    if (!listener) {
      locked = false;
      return;
    }
    listener()
      .then(
        () => { runNextListener(); },
        () => { runNextListener(); }
      );
  };

  const run = listener => new Promise((resolve, reject) => {
    listeners.unshift(() => {
      return listener().then(
        () => { resolve(); },
        error => { reject(error); }
      );
    });
    if (!locked) {
      locked = true;
      runNextListener();
    }
  });

  return run;

};

module.exports.createMutexLock;