Understanding setImmediate()

setImmediate() schedules a callback for the check phase of a future event-loop iteration:

setImmediate(() => {
  console.log('immediate')
})

It means “run after the current work yields and the loop reaches the check phase,” not “run immediately” and not “run after exactly zero milliseconds.”

Compare the three queues:

• process.nextTick() runs after the current JavaScript operation, before the event loop continues to another phase.
• setTimeout(callback, 0) schedules a timer after a minimum delay threshold. The operating system and other work can delay it.
• setImmediate() runs in the check phase, after the poll phase used for I/O callbacks.

From the top level of a script, the relative order of a zero-delay timeout and an immediate should not be treated as a contract:

setTimeout(() => console.log('timeout'), 0)
setImmediate(() => console.log('immediate'))

Inside an I/O callback, the immediate is scheduled for the check phase following that poll work and normally runs before a newly scheduled zero-delay timer:

import { readFile } from 'node:fs'

readFile(import.meta.filename, () => {
  setTimeout(() => console.log('timeout'), 0)
  setImmediate(() => console.log('immediate'))
})

process.nextTick() is not an ordinary event-loop phase. Recursively scheduling next-tick callbacks can starve timers and I/O because Node drains that queue before continuing. Use it for narrow API-ordering needs, not for breaking up large computations.

An Immediate normally keeps the process alive. Calling .unref() allows the process to exit if that callback is the only remaining work:

setImmediate(() => console.log('may run')).unref()

Ordering details can evolve with Node and its event-loop implementation. Write code around documented phase relationships, never a race observed once on your machine.

Your action is to run the top-level and I/O examples ten times on your Node version. Record the output, then add five recursive process.nextTick() calls and explain why an unbounded version would delay other work.