callback_interface.js

/* There are basically 3 solutions for async programming, and 2 of which are with related to Promises. */

/* Promises Notation (#1 & #2) */
// The Promise definition
function timedAction() {
  return new Promise(resolve => {
    setTimeout(() => { resolve('done') }, 2000)
  })
}

// The Promise execution definitions, 1 and 2 are equivalent:
// 1 - Pseuo procedurally with 'await' (error handling: try/catch blocks)
async function saySomething() {
  try {
    const msg = await timedAction()
    console.log('Message: "%s"', msg)
  } catch (err) {
    console.error(err);
  }
}

// 2 - 'Then/Catch' method chaining (error handling: then/catch methods)
function saySomething() {
  timedAction()
    .then(msg => console.log('Message: "%s"', msg))
    .catch(console.error)
}

/* Older Callback Notation (#3) */
// The callback definition
function timedActionCB(cb) {
  setTimeout(() => { cb(null, 'done') }, 2000)
}

// 3 - The callback execution definition (error handling: if/else blocks)
function saySomething() {
  timedActionCB(function(err, msg) {
    if (err) {
      console.error(err)
    } else {
      console.log('Message: "%s"', msg)
    }
  })
}

/* Execution Result (Regardless of implementation) */
saySomething() // Message: "done" <-- after 2 seconds