Event Loop & Microtasks

What is the output sequence?

Promise.resolve()
  .then(() => console.log(1))
  .then(() => console.log(2));

Promise.resolve()
  .then(() => console.log(3))
  .then(() => console.log(4));

1, 2, 3, 4
1, 3, 2, 4
The output sequence demonstrates microtask chaining behavior: 1) First .then() callbacks from both chains are microtasks in the same tick (1, 3), 2) Second .then() callbacks form a new microtask queue (2, 4), 3) Within each microtask level, promises are handled in creation order, 4) Each .then() creates a new microtask, 5) All microtasks complete before any macrotasks, 6) This pattern is essential for understanding Promise execution order.
3, 4, 1, 2
1, 2, 4, 3

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