Understanding process.nextTick()

As you try to understand the Node.js event loop, one important part of it is process.nextTick(). Every time the runtime calls back into JavaScript for an event, we call it a tick.

When we pass a function to process.nextTick(), we instruct the engine to invoke this function immediately after the current operation completes, before moving to the next phase in the event loop:

process.nextTick(() => {
  // do something
});

The event loop is busy processing the current function code. When this operation ends, the JS engine runs all the functions passed to nextTick calls during that operation.

It's the way we can tell the JS engine to process a function asynchronously (after the current function), but as soon as possible, not queue it.

Calling setTimeout(() => {}, 0) will execute the function at the end of next tick, much later than when using nextTick() which prioritizes the call and executes it just before the beginning of the next tick.

Use nextTick() when you want to make sure that in the next event loop iteration that code is already executed.

An Example of the order of events:

console.log('Hello => number 1');

setImmediate(() => {
  console.log('Running before the timeout => number 3');
});

setTimeout(() => {
  console.log('The timeout running last => number 4');
}, 0);

process.nextTick(() => {
  console.log('Running at next tick => number 2');
});

Example output:

Hello => number 1
Running at next tick => number 2
Running before the timeout => number 3
The timeout running last => number 4

The exact output may differ from run to run.