Node.js: inspect what's left in the event loop that's preventing the script from exiting naturally

You can call process._getActiveRequests() to get a list of active I/O requests and process._getActiveHandles() to get a list of open handles/file descriptors.

The wtfnode package utilizes the tools mentioned by @mscdex but presents in a more developer-friendly way. I highly recommend this for tracking down these annoyances.

const wtf = require('wtfnode');

// ...

wtf.dump();

Node.js, starting from version 8, provides a handy module to retrieve this kind of information.

async_hooks: an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application.