Why is the behavior of setTimeout(0) and setImmediate() undefined when used in the main module?

Question

Take the following code taken from the nodejs event loop documentation :

// timeout_vs_immediate.js
setTimeout(() => {
  console.log('timeout');
}, 0);

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

According to the documentation :

For example, if we run the following script which is not within an I/O cycle (i.e. the main module), the order in which the two timers are executed is non-deterministic, as it is bound by the performance of the process.

Why is the above statement true ? Is it because the nodejs runtime actually employs more than one thread to pick out the callbacks that have to be executed.

What my intuition says: there are two threads that execute callbacks for setTimeout and setImmediate so when both of them are available this leads to a race condition, and thus the output will be non-deterministic.

Is it correct ? Or is there any other reason for which this is non-deterministic ?

Answer 1

Essentially, two things happen:

1. setTimer(0..) gets converted into setTimer(1..)

2. before the (next) event loop tick begins, node/libuv has to perform a clock_gettime() to get the current time from the system. The time taken for this system call is non deterministic as it depends on the system load at that time. Now, if clock_gettime() took more than 1ms, the setTimer callback will run (#), else event loop continues to next phase (##).

   • In case (#), setTimeout(0,..) callback is run before setImmediate()
   • In case (##), it is otherwise.

Reference: https://github.com/nodejs/help/issues/392#issuecomment-305969168