index.js

const handledEvents = ["SIGINT", "SIGTERM", "SIGQUIT"];

const dependencyTree = new Map(); // name => [dependency name, ...]
const handlers = new Map(); // name => [handler, ...]
const shutdownErrorHandlers = [];

/**
 * Gracefully terminate application's modules on shutdown.
 * @param {string} [name] - Name of the handler.
 * @param {array} [dependencies] - Which handlers should be processed first.
 * @param {function} handler - Async or sync function which handles shutdown.
 */
module.exports.onShutdown = function (name, dependencies, handler) {
  handler =
    typeof name === "function"
      ? name
      : typeof dependencies === "function"
      ? dependencies
      : handler;
  dependencies =
    name instanceof Array
      ? name
      : dependencies instanceof Array
      ? dependencies
      : [];
  name = typeof name === "string" ? name : Math.random().toString(36);

  if (dependencies.reduce((acc, dep) => acc || testForCycles(dep), false)) {
    throw new Error(
      `Adding shutdown handler "${name}" will create a dependency loop: aborting`
    );
  }

  dependencyTree.set(
    name,
    Array.from(new Set((dependencyTree.get(name) || []).concat(dependencies)))
  );
  if (!handlers.has(name)) {
    handlers.set(name, []);
  }
  handlers.get(name).push(handler);
};

/**
 * Optional export to handle shutdown errors.
 * @param {function} callback
 */
module.exports.onShutdownError = function (callback) {
  shutdownErrorHandlers.push(callback);
};

async function shutdown(name, promisesMap) {
  if (promisesMap.has(name)) {
    return await promisesMap.get(name);
  }

  const nodeCompletedPromise = (async function () {
    const dependencies = dependencyTree.get(name) || [];

    // Wait for all dependencies to shut down.
    await Promise.all(dependencies.map((dep) => shutdown(dep, promisesMap)));

    // Shutdown this item.
    const allHandlers = handlers.get(name) || [];
    if (allHandlers.length) {
      await Promise.all(allHandlers.map((f) => f()));
    }
  })();

  promisesMap.set(name, nodeCompletedPromise);
  await nodeCompletedPromise;
}

let shuttingDown = false;
handledEvents.forEach((event) =>
  process.removeAllListeners(event).addListener(event, () => {
    if (shuttingDown) {
      return;
    }
    shuttingDown = true;

    // Get all unreferenced nodes.
    const unreferencedNames = getAllUnreferencedNames();

    const visited = new Map();
    Promise.all(unreferencedNames.map((name) => shutdown(name, visited)))
      .then(() => exit(0))
      .catch((e) => {
        Promise.all(shutdownErrorHandlers.map((f) => f(e)))
          .then(() => exit(42759))
          .catch(() => exit(42758));
      });
  })
);

// -------- Utility functions -------- \\

function testForCycles(name, visitedSet = new Set()) {
  // Return true if the cycle is found.
  if (visitedSet.has(name)) {
    return true;
  }
  visitedSet.add(name);
  // If any of the cycles found in dependencies, return true.
  return (dependencyTree.get(name) || []).reduce(
    (acc, name) => acc || testForCycles(name),
    false
  );
}

function getAllUnreferencedNames() {
  const allNodes = new Set(Array.from(dependencyTree.keys()));
  Array.from(dependencyTree.values()).forEach((deps) =>
    deps.forEach((dep) => allNodes.delete(dep))
  );
  return Array.from(allNodes);
}

/* STUBBED - DO NOT EDIT */
function exit(code) {
  process.exit(code);
}