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process.nextTick(): schedules work after the current operation, before the event loop proceeds. Node maintains a separate queue for it.
setImmediate(): schedules a callback in the check phase after poll.
setTimeout(fn, 0): schedules a timer with a minimum threshold, not a promise to run immediately or at an exact time.
In ordinary CommonJS synchronous code, nextTick callbacks run before queued Promise microtasks. ES module top-level evaluation and await continuations already run in a microtask context, so newly queued Promise callbacks can run before nextTick work. Do not apply one universal priority diagram to every context.
Trap: when both are scheduled from the main module, the order of a zero-delay timer and setImmediate is not guaranteed. When both are scheduled inside the same I/O callback, setImmediate runs first. Recursive nextTick or microtask work can starve normal loop progress.
Source: Node.js scheduling examples.
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