Execution of Promise Chain in Node.js

README.md

Execution of Promise Chain in Node.js

After reading The Node.js Event Loop, Timers, and process.nextTick()...

Q. I write a lot of promise chains in applications. How is a promise chain executed? Is it performant?

The experiment shows:

• Chained promises are eagerly executed before proceeding to the next phase.
• Promise chaining is a bit slower than synchronous execution, but not too much.
• (Also, chained process.nextTick() are eagerly executed before proceeding to the next phase.)

event-loop.js

setTimeout(() => {
  process.nextTick(() => {
    console.log('nextTick 0');
    console.time('nextTick 1');
    process.nextTick(() => {
      console.timeEnd('nextTick 1');
      console.time('nextTick 2');
      process.nextTick(() => {
        console.timeEnd('nextTick 2');
      });
    });
  });

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

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

  Promise.resolve(1)
    .then((n) => {
      console.log('Promise 0');
      console.time('Promise 1');
      return n + 1;
    })
    .then((n) => {
      console.timeEnd('Promise 1');
      console.time('Promise 2');
      return Promise.resolve(n + 1);
    })
    .then((n) => {
      console.timeEnd('Promise 2');
      console.time('Promise 3');
      return Promise.resolve(n + 1);
    })
    .then((n) => {
      console.timeEnd('Promise 3');
    });

  console.time('sync 0');
  const n = 1;
  console.timeEnd('sync 0');
  console.time('sync 1');
  const nn = n + 1;
  console.timeEnd('sync 1');
  console.time('sync 2');
  const nnn = nn + 1;
  console.timeEnd('sync 2');
});

output.txt

$ node -v
v10.4.0
$ node index.js
sync 0: 0.146ms
sync 1: 0.004ms
sync 2: 0.004ms
nextTick 0
nextTick 1: 0.387ms
nextTick 2: 0.031ms
Promise 0
Promise 1: 0.029ms
Promise 2: 0.025ms
Promise 3: 0.018ms
setImmediate
setTimeout