Great examples to understand the differences of setImmediate, setTimeout and nextTick()

event-loop-examples.js

/**
 * setImmediate callbacks are fired off the event loop, once per iteration in the order that they were queued. 
 * So on the first iteration of the event loop, callback A is fired.
 * Then on the second iteration of the event loop, callback B is fired, then on the third iteration of the event loop callback C is fired, etc.
 * This prevents the event loop from being blocked and allows other I/O or timer callbacks to be called in the mean time (as is the case of the 0ms timeout, which is fired on the 1st or 2nd loop iteration).
 */

setImmediate(function A() {
  setImmediate(function B() {
    console.log(1);
    setImmediate(function D() { console.log(2); });
    setImmediate(function E() { console.log(3); });
  });
  setImmediate(function C() {
    console.log(4);
    setImmediate(function F() { console.log(5); });
    setImmediate(function G() { console.log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

/**
 * nextTick callbacks, however, are always fired immediately after the current code is done executing and BEFORE going back to the event loop. 
 * In the nextTick example, we end up executing all the nextTick callbacks before ever returning to the event loop.
 * Since setTimeout's callback will be called from the event loop, the text 'TIMEOUT FIRED' will not be output until we're done with every nextTick callback.
 * Thats one of the reason why you should never call nextTick recursively if you want to process IO operations.
 */
process.nextTick(function A() {
  process.nextTick(function B() {
    console.log(1);
    process.nextTick(function D() { console.log(2); });
    process.nextTick(function E() { console.log(3); });
  });
  process.nextTick(function C() {
    console.log(4);
    process.nextTick(function F() { console.log(5); });
    process.nextTick(function G() { console.log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)

/**
 * setTimeout is simply like calling the function after delay has finished. Whenever a function is called it is not executed immediately, 
 * but queued so that it is executed after all the executing and currently queued eventhandlers finish first. 
 * setTimeout(,0) essentially means execute after all current functions in the present queue get executed. No guarantees can be made about how long it could take.
 */

setTimeout(function A() {
  setTimeout(function B() {
    console.log(1);
    setTimeout(function D() { console.log(2); });
    setTimeout(function E() { console.log(3); });
  });
  setTimeout(function C() {
    console.log(4);
    setTimeout(function F() { console.log(5); });
    setTimeout(function G() { console.log(6); });
  });
});

setTimeout(function timeout() {
  console.log('TIMEOUT FIRED');
}, 0)