Microtask Lab

See why promise callbacks run before setTimeout(…, 0).

Phase 10 taught the task queue (macrotasks) for timers. Promise handlers use a separate microtask queue. After the current stack empties, the engine drains microtasks before the next timer task. So: sync → promise .then → setTimeout.

Call stack

    Microtasks

      Task queue

        Press Run — expect: A, C, then promise B, then timeout D.

        Idle.

        Important JavaScript

        console.log("A");

Promise.resolve().then(function () {
  console.log("B"); // microtask
});

console.log("C");

setTimeout(function () {
  console.log("D"); // macrotask
}, 0);

// Prints: A, C, B, D