Node.js Eventloop Timers and NextTick

nodejs-eventloop-timers-and-nexttick.js

/**
 * nextTick vs setImmediate, visual explanation
 *
 * https://stackoverflow.com/questions/17502948/nexttick-vs-setimmediate-visual-explanation
 * https://nodejs.dev/learn/understanding-process-nexttick
 */

// SETIMMEDIATE

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

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

// 'TIMEOUT FIRED' 1 4 2 3 5 6
// OR
// 1 'TIMEOUT FIRED' 4 2 3 5 6


// NEXTTICK

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

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

// 1 4 2 3 5 6 'TIMEOUT FIRED'

/**
 * The only real difference is that multiple nextTick may fire in one loop iteration (depending on maxTickDepth),
 * while setImmediate fires once per iteration.
 */