The answer
Microtasks are promise callbacks and process.nextTick, which run after the current operation and before the next macrotask. Macrotasks are setTimeout, setInterval, setImmediate, and I/O callbacks, which run one per event loop phase.
Node.js Event Loop Order of Execution Explained With Code
What are microtasks vs macrotasks in Node.js?
More FAQs in Node.js Event Loop Order of Execution Explained With Code
Sync code runs first, then process.nextTick callbacks, then promise microtasks, then macrotasks (timers, I/O callbacks, setImmediate). Microtasks always run before the next macrotask, which is the key rule.
Because promises are microtasks, which run after the current operation but before the next macrotask. setTimeout is a macrotask. So in console.log('1'); setTimeout(fn); promise.then(fn); console.log('4'), the order is 1, 4, then the promise, then setTimeout.
process.nextTick callbacks. They run before promise microtasks, both before macrotasks. So console.log('1'); nextTick(fn); promise.then(fn) outputs 1, nextTick, promise. This is a Node.js-specific behavior.
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