Difference between microtask and macrotask within an event loop context

by

One go-around of the event loop will have exactly one task being processed from the macrotask queue (this queue is simply called the task queue in the WHATWG specification).
After this macrotask has finished, all available microtasks will be processed, namely within the same go-around cycle. While these microtasks are processed, they can queue even more microtasks, which will all be run one by one, until the microtask queue is exhausted.

What are the practical consequences of this?

If a microtask recursively queues other microtasks, it might take a long time until the next macrotask is processed. This means, you could end up with a blocked UI, or some finished I/O idling in your application.

However, at least concerning Node.js’s process.nextTick function (which queues microtasks), there is an inbuilt protection against such blocking by means of process.maxTickDepth. This value is set to a default of 1000, cutting down further processing of microtasks after this limit is reached which allows the next macrotask to be processed)

So when to use what?

Basically, use microtasks when you need to do stuff asynchronously in a synchronous way (i.e. when you would say perform this (micro-)task in the most immediate future).
Otherwise, stick to macrotasks.

Examples

macrotasks: setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O, UI rendering
microtasks: process.nextTick, Promises, queueMicrotask, MutationObserver

More Related Contents:

  1. Removing nested promises
  2. Chained promises not passing on rejection
  3. Issue in returning data retrieved from DB queries called in the loop
  4. Is “.then(function(a){ return a; })” a no-op for promises?
  5. Asynchronous exception handling with bluebird promises
  6. Is Node.js native Promise.all processing in parallel or sequentially?
  7. Using async/await with a forEach loop
  8. Object method with ES6 / Bluebird promises
  9. Node JS Promise.all and forEach
  10. Understanding promises in Node.js
  11. Can’t throw error from within an async promise executor function
  12. How can I chain these functions together with promises?
  13. Getting a UnhandledPromiseRejectionWarning when testing using mocha/chai
  14. How can i return status inside the promise?
  15. Node exits without error and doesn’t await promise (Event callback)
  16. Nodejs express and promises not doing what I expect
  17. How do I handle errors with promises?
  18. Placement of catch BEFORE and AFTER then
  19. What is the relationship between event loop and Promise [duplicate]
  20. Why is `.catch(err => console.error(err))` discouraged?
  21. How to do promise.all for array of array of promises?
  22. Do Javascript promises block the stack
  23. What happens if i reject / resolve multiple times in Kriskowal’s q?
  24. What is the difference between “event loop queue” and “job queue”?
  25. Using mongoose promises with async/await
  26. How to structure nested Promises
  27. What is the intention behind clause 2.2.4 of Promise/A+ spec?
  28. Limit concurrency of pending promises
  29. Node.js: When to use Promises vs Callbacks
  30. JavaScript asynchronous programming: promises vs generators

Leave a Comment