event-loop-order-of-code

test.js

setTimeout(() => { console.log('timeout'); }, 0); 
setImmediate(() => { console.log('immediate'); });

const fs = require('fs');

fs.readFile(__filename, () => {
  setTimeout(() => {
    console.log('timeout');
  }, 0);
  setImmediate(() => {
    console.log('immediate');
  });
});

if you run the test.js which the two firsts scripts arent within the I/O cycle. the order in which the two timers are executed is non-deterministic

The main advantage to using setImmediate() over setTimeout() is setImmediate() will always be executed before any timers if scheduled within an I/O cycle