Node.js Event Loop Order of Execution Explained With Code

Node.js Event Loop Order of Execution Explained With Code

The event loop's order of execution can be confusing. Here is it explained with concrete code examples.

Microtasks Run Before Macrotasks

Promise callbacks (.then, .catch, async/await after the await) are microtasks. They run after the current operation and before the next macrotask, including before setTimeout(0) and setImmediate.

The Code Example

console.log('1'); setTimeout(() => console.log('2'), 0); Promise.resolve().then(() => console.log('3')); console.log('4');

The output is 1, 4, 3, 2. Sync code runs first (1, 4), then microtasks (3), then macrotasks (2). This is the key: microtasks run before the next macrotask.

process.nextTick vs Promises

process.nextTick callbacks run before promise callbacks, both before macrotasks. So: console.log('1'); process.nextTick(() => console.log('2')); Promise.resolve().then(() => console.log('3')); outputs 1, 2, 3. nextTick runs before promises.

Why This Matters

If you do not understand the order, you cannot predict when async code runs, which causes bugs and confusion. The order is deterministic: sync code, then microtasks (nextTick then promises), then macrotasks (timers, I/O callbacks, setImmediate).

Phases Within a Tick

Within a tick, the order is: sync code completes, then nextTick queue drains, then microtask (promise) queue drains, then the next phase of the event loop runs (timers, then poll, then check).

The Takeaway

The event loop order is: sync code, then process.nextTick, then promise microtasks, then macrotasks (timers, I/O, setImmediate). Understanding this order is essential for predicting when async code runs and debugging timing bugs.