Event Loop Lab

See how the call stack, Web APIs, task queue, and event loop work together.

Synchronous code runs on the call stack immediately. Timers are handed to a Web API. When the delay ends, the callback joins the task queue. The event loop moves a queued callback onto the stack only after the stack is empty.

Call stack

    Web APIs

      Task queue

        Press Run to watch order: A, C, then B.

        Idle.

        Important JavaScript

        console.log("A");

setTimeout(function () {
  console.log("B");
}, 0);

console.log("C");
// Prints: A, C, then B
// B waited in the task queue until the stack was clear.