What is queueMicrotask() and when should you use it?

queueMicrotask(fn) adds a callback directly to the microtask queue — the same queue that Promise callbacks use.

// Functionally equivalent:
Promise.resolve().then(() => console.log('A'));
queueMicrotask(() => console.log('B'));
// A, B — FIFO within the microtask queue

console.log('sync');
queueMicrotask(() => console.log('microtask'));
setTimeout(() => console.log('macrotask'), 0);
console.log('sync end');
// Order: sync → sync end → microtask → macrotask

Advantages over Promise.resolve().then():

• No Promise overhead — slightly more performant
• More explicit — clearly states "schedule as microtask"
• Doesn't create a Promise chain

// Real use case: batch state updates
let pending = false;
function scheduleRender() {
  if (pending) return;
  pending = true;
  queueMicrotask(() => {
    pending = false;
    renderDOM(); // runs once after all sync mutations
  });
}