print-leaked-events.js

// This has been updated. You'll have to go back in time in the gist history to
// see older versions.

const { writeSync } = require("fs")
const async_hooks = require("async_hooks")

async function printLeakedEvents(f) {
  // Track all active event IDs
  const eventIDs = new Set()

  // Set up an async hook to increment and decrement the counter
  const asyncHook = async_hooks.createHook({
    init: (asyncID) => {
      eventIDs.add(asyncID)
    },
    after: (asyncID) => {
      eventIDs.delete(asyncID)
    },
  });

  // run the function with the async hook enabled
  asyncHook.enable()
  try {
    await f()
  } finally {
    asyncHook.disable()
  }

  // print the number of hooks (using writeSync to be 100% sure that we don't
  // create any new events)
  writeSync(1, eventIDs.size + "\n")
}

async function main() {
  // do no async stuff
  await printLeakedEvents(async () => 0)

  // do no async stuff again to make sure it's reliable
  await printLeakedEvents(async () => 0)

  // leak a promise
  await printLeakedEvents(async () => {
    // unresolved promise
    new Promise(function(resolve, reject) { setTimeout(resolve, 1000); })
  })

  // create a promise and resolve it correctly, including `then` and `await`
  await printLeakedEvents(async () => {
    // unresolved promise
    await new Promise(function(resolve, reject) {
      setTimeout(resolve, 1000);
    }).then(() => {
      // do nothing, but we do want a "then"
    })
  })
}

main()

// Output:
// 6
// 6
// 8
// 8

// The questions:
//
// 1. Why are 6 events created ("init"ed) but not resolved when I call a
// synchronous function?
//
// 2. Why do I see a net event change of 8 regardless of whether I correctly
// `await` the promise with a `then`, vs. simply leaking it?

a more complete version from @wolever that seems to actually work: https://gist.github.com/wolever/d95c7143af5789161868c97a0ce9e2fe