Tasks, microtasks, queues and schedules

Tasks, microtasks, queues and schedules

When I told my colleague Matt Gaunt I was thinking of writing a piece on microtask queueing and execution within the browser's event loop, he said "I'll be honest with you Jake, I'm not going to read that". Well, I've written it anyway, so we're all going to sit here and enjoy it, ok?

Actually, if video's more your thing, Philip Roberts gave a great talk at JSConf on the event loop - microtasks aren't covered, but it's a great introduction to the rest. Anyway, on with the show…

Take this little bit of JavaScript:

console.log('script start');

setTimeout(function () {
  console.log('setTimeout');
}, 0);

Promise.resolve()
  .then(function () {
    console.log('promise1');
  })
  .then(function () {
    console.log('promise2');
  });

console.log('script end');

In what order should the logs appear?