Weak Timeout and Event Loop References

A node will keep running as long as there is a strong reference in the Event Loop.

Sometimes you need to have something happen on a timer, but you don't want that timer to keep the process alive.

For that you can use unref():

setTimeout(fn, ms).unref();

`unref()` all the things

Other than setTimeout and setInterval, there are a number of Node APIs that support unref() to create weak references in the event loop.

This can be useful for servers, database connections, child processes, and many other things.

setTimeout(fn, ms).unref();
setInterval(fn, ms).unref();
setImmediate(fn).unref();

child_process.fork(modulePath).unref();
child_process.spawn(command).unref();

net.createServer(options).unref();
http.createServer(options).unref();

Docs:

If you're using the serverless framework for lambda functions or some such, you can set callbackWaitsForEmptyEventLoop = false to work around strong references.

Example

function weakBackgroundTask() {
    console.log('interval weak reference (50ms)');
}

function backgroundTask() {
    console.log('releasing strong reference');
    // program will exit now
}

console.log('creating an interval weak reference (every 50ms)');
setInterval(weakBackgroundTask, 50).unref();

console.log('creating a one-time strong reference (in 500ms)');
setTimeout(backgroundTask, 500);