Predict the output: setTimeout, a promise, process.nextTick and plain logs together

Picture a railway counter. The person at the window finishes the current customer completely (synchronous code). Before calling the next customer from the queue, they clear two small priority slips: first the nextTick slip, then the promise slip. Only then do they call the next ordinary customer, which is a timer or I/O callback.

console.log("A");

setTimeout(() => console.log("B: timeout"), 0);

Promise.resolve().then(() => console.log("C: promise"));

process.nextTick(() => console.log("D: nextTick"));

console.log("E");

// prints:
// A
// E
// D: nextTick
// C: promise
// B: timeout

Step by step:

1. A and E are synchronous, so they print first.
2. When the main script ends, Node drains the nextTick queue, so D prints.
3. Then the promise microtask queue drains, so C prints.
4. The event loop then reaches the timers phase and B prints.

In an ES module file the order of D and C can differ, because the module itself runs inside a promise job. That is why most examples use CommonJS.

• Thinking setTimeout(fn, 0) runs immediately.
• Starving the loop with recursive nextTick calls, which blocks I/O.