process.nextTick()

process.nextTick(callback[, arg][, ...])

callback <Function>

Once the current event loop turn runs to completion, call the callback function.

This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

console.log('start');
process.nextTick(() => {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

This is important in developing APIs where you want to give the user the chance to assign event handlers after an object has been constructed, but before any I/O has occurred.

function MyThing(options) {
  this.setupOptions(options);
 
  process.nextTick(() => {
    this.startDoingStuff();
  });
}
 
var thing = new MyThing();
thing.getReadyForStuff();
 
// thing.startDoingStuff() gets called now, not before.

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

// WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
  if (arg) {
    cb();
    return;
  }
 
  fs.stat('file', cb);
}

This API is hazardous. If you do this:

maybeSync(true, () => {
  foo();
});
bar();

then it's not clear whether foo() or bar() will be called first.

This approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }
 
  fs.stat('file', cb);
}

Note: the nextTick queue is completely drained on each pass of the event loop before additional I/O is processed. As a result, recursively setting nextTick callbacks will block any I/O from happening, just like a while(true); loop.

Links:

https://nodejs.org/dist/latest-v5.x/docs/api/process.html#process_process_nexttick_callback_arg

doc_Nodejs

2025-01-10 15:47:30

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