What is the difference between regular tasks and micro-tasks?

Regular tasks (macrotasks) are executed after the current Event Loop macro cycle completes. Microtasks are executed after the current function finishes but before the next macro cycle begins.

Regular tasks include: scripts, timers (setTimeout, setInterval), input/output operations. Microtasks include: promises (.then(), .catch(), .finally()), process.nextTick(), queueMicrotask(), MutationObserver (in browsers).

Example execution order:

console.log('Start'); // Regular task 1 (initial script)

setTimeout(() => {
  console.log('setTimeout'); // Regular task 2
}, 0);

Promise.resolve()
  .then(() => {
    console.log('Promise 1'); // Microtask 1
  })
  .then(() => {
    console.log('Promise 2'); // Microtask 2
  });

process.nextTick(() => {
  console.log('nextTick'); // Microtask 3 (executes before promises)
});

console.log('End'); // Regular task 1 (script continuation)

Output order:

Start
End
nextTick
Promise 1
Promise 2
setTimeout

Main differences:

Characteristic	Regular tasks (Macrotasks)	Microtasks
Execution queue	Executed after all microtasks of the current cycle.	Executed after the current operation but before the next regular task.
Sources	setTimeout, setInterval, I/O, UI rendering.	Promise.then, process.nextTick, queueMicrotask.
Priority	Lower	Higher