What is the difference between process.nextTick and setImmediate?

Both schedule callbacks asynchronously but at different phases:

process.nextTick

• Runs before the event loop continues.
• Executes after current operation, before I/O callbacks.
• Can starve I/O if used recursively.

setImmediate

• Runs in the check phase of the event loop.
• After I/O callbacks, before timers in many cases.

Rule of thumb: use setImmediate for deferring work to the next event loop iteration; use nextTick when you need to run before any I/O callbacks.