Guess the output #1

Guess the output #1

Question:

const promise = new Promise((resolve, reject) => {
  console.log(1);
  setTimeout(() => {
    console.log("timerStart");
    resolve("success");
    console.log("timerEnd");
  }, 0);
  console.log(2);
});

promise.then((res) => {
  console.log(res);
});

console.log(4);

Output:

1
2
4
timerStart
timerEnd
success

Explanation:

1. Immediate Execution:
   • console.log(1) is executed, printing 1.
   • console.log(2) is executed, printing 2.
2. Promise Creation:
   • A Promise is created, but it doesn't resolve immediately. The setTimeout function schedules the callback to run after 0 milliseconds (as soon as possible, but after the current execution stack is cleared).
3. Synchronous Code Continues:
   • console.log(4) is executed, printing 4.
4. Event Loop & Microtask Queue:
   • After the synchronous code execution finishes, the event loop picks up the callback from setTimeout. This prints timerStart.
   • The resolve("success") call is executed, but the then callback is queued in the microtask queue.
   • console.log("timerEnd") is executed, printing timerEnd.
5. Microtask Execution:
   • The event loop then processes the microtask queue, where the then callback is executed, printing success.