What is the order of execution in the Node.js event loop?
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.
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More FAQs in Node.js Event Loop Order of Execution Explained With Code
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.
Because if you do not understand the order, you cannot predict when async code runs, which causes bugs and confusion. The order is deterministic: sync, nextTick, promises, then macrotasks. Understanding it is essential for debugging timing bugs.
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