What runs before promises in the Node.js event loop?
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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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.
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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