Avoid Creating Unnecessary Promises!!

1. bad.js

const { promisify } = require('util')
const timeout = promisify(setTimeout)

async function bar() { await timeout(1000) }

// The idea here is that we don't know if foo
// is going to return a Promise or not, so just
// in case, force it to return a Promise.
// Unfortunately, doing it this way creates
// quite a few additional unnecessary Promises
//
async function foo(a) {
  return a || await bar()
}

foo(true).then(console.log)
foo(false).then(console.log)

// Run this code with
// node --trace-event-categories node.async_hooks bad.js
//
// Then open chrome://tracing and load the `node_trace.1.log`
// to see just how many Promises are created

2. better.js

const { promisify } = require('util')
const timeout = promisify(setTimeout)

async function bar() { await timeout(1000) }

// By using Promise.resolve() and not forcing
// foo to be async, we achieve the exact same
// result with fewer Promises created. However,
// that Promise.resolve() is still creating a
// completely unnecessary Promise in this case
// leading to unnecessary scheduling delay and
// memory allocation. Bad foo!
function foo(a) {
  return a ? Promise.resolve(a) : bar()
}

foo(true).then(console.log)
foo(false).then(console.log)

// Run this code with
// node --trace-event-categories node.async_hooks better.js
//
// Then open chrome://tracing and load the `node_trace.1.log`
// to see just how many Promises are created

3. best.js

const { promisify } = require('util')
const timeout = promisify(setTimeout)

async function bar() { await timeout(1000) }

// WHAT!? Callbacks *with* my Promises, you
// must be joking!!? Nope... with this approach
// you don't have to even think about whether
// you're dealing with a Promise or not. When
// the data is available, sync or async, the
// callback will be invoked and no unnecessary
// Promises will be created.
function foo(a, cb) {
  if (a) cb(null, a)
  else {
    bar().then(cb.bind(null, null))
         .catch(cb.bind(null))
  }
}

foo(true, console.log)
foo(false, console.log)

// Run this code with
// node --trace-event-categories node.async_hooks best.js
//
// Then open chrome://tracing and load the `node_trace.1.log`
// to see just how many Promises are created