Predict the output: event loop ordering with sync code, Promises, and setTimeout

What prints and in what order?

console.log('A')

setTimeout(() => console.log('B'), 0)

Promise.resolve()
  .then(() => console.log('C'))
  .then(() => console.log('D'))

console.log('E')

Answer: A → E → C → D → B

Step by step

1. Sync phase (call stack): A, schedule macrotask B, schedule microtasks for Promise chain, E.
2. Microtask drain: First .then runs → C. That schedules another microtask → D.
3. Microtask drain continues: D runs.
4. Macrotask: setTimeout callback → B.

Rules to remember

• Synchronous code always runs first.
• The microtask queue is fully drained before the next macrotask.
• New microtasks scheduled during microtask processing run in the same turn.
• setTimeout(fn, 0) is not instant — it waits for the stack to clear and all microtasks.

Variant trap: If the second .then did setTimeout(() => log('F'), 0), F would print after B because it is scheduled in a later macrotask turn.