The Event Loop

JavaScript jobs run to completion on a call stack. Hosts queue tasks for timers, events, and I/O; Promise reactions and queueMicrotask use the microtask queue.

Ordering

console.log("start");
setTimeout(() => console.log("task"), 0);
Promise.resolve().then(() => console.log("microtask"));
console.log("end");
// start, end, microtask, task

After a task, the host drains microtasks before another task and usually before rendering. Recursive microtasks can starve input/rendering; long synchronous work blocks everything. Split CPU work, yield appropriately, or move suitable computation to workers. Node has related phases and additional queues, so exact host ordering rules matter.