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);
         


        

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