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As per node.js documentation on "Understanding process.nextTick()"(https://nodejs.dev/learn/understanding-process-nexttick):

Every time the event loop takes a full trip, we call it a tick.

what does full trip mean?

In the below example code, since the process.nextTick() is added inside zoo() which is called inside foo(), shouldn't the function test("called from tick inside zoo") get called before boo()?

function test(msg) {
  console.log(msg);
}

function zoo() {
  console.log("Hello, I am zoo");

  process.nextTick(() => test("called from tick inside zoo"));
}

function foo() {
  console.log("Hello, I am foo");
  setTimeout(() => test("called from setTimeout foo"), 0);
  zoo();

}

function boo() {
  console.log("I am boo")
}

foo()
boo()

OUTPUT:

Hello, I am foo
Hello, I am zoo
I am boo
called from tick inside zoo
called from setTimeout foo

Also, in the same documentation page:

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.

Does that mean that process.nextTick() basically ensures that the function passed gets priority over the queue(both message/job queue) but not in between the function calls within the call stack(as in foo() and boo() in the above example)?

If the above example does not apply the concept correctly. Please help me understand process.nextTick() with a simple example.

Heretic Monkey
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Sowmya
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    Very similar: [Why and when to use process.nextTick?](https://stackoverflow.com/q/40629456/215552) and pretty much a duplicate of [When is process.nextTick() in the NodeJS event loop called?](https://stackoverflow.com/q/64264617/215552) – Heretic Monkey May 05 '22 at 20:40
  • "*what does full trip mean?*" - roundtrip. One iteration of the event *loop*. – Bergi May 05 '22 at 22:09

1 Answers1

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From the link in the question, this explains why:

When we pass a function to process.nextTick(), we instruct the engine to invoke this function at the end of the current operation, before the next event loop tick starts:

Why does "called from tick inside zoo" after "I am boo"?

boo() is called synchronously. This means it will be called in the current tick process.nextTick() will be called after the current tick:

we instruct the engine to invoke this function at the end of the current operation

Why is process.nextTick() callback called before setTimeout() callback?

Timeout callbacks will be called in a future tick, it could be the next tick or some ticks after the next tick. From the guide it says that process.nextTick() callbacks in between the current tick and the next tick:

before the next event loop tick starts

programmerRaj
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