Asynchronous Javascript

main.js

const start = require('./start')
const run = require('./run')


// INCORRECT approach:

// start doesn't return anything immediately, so this variable
//  will be undefined.
const willBeUndefined = start((myPet) => {
    // We can log the pet which start creates
    console.log('myPet = ' + JSON.stringify(myPet))
})

// If we call run outside of the start callback it will run
//  before start finishes. We can't guarantee that start
//  has completed unless we are inside of the start callback.
run(willBeUndefined)


// CORRECT approach:

// start receives a callback which is called once start is completed.
// We call run() in this callback to run after start
start((myPet) => {
    run(myPet)
})

start.js

const rl = require('readline')

function start(callback) {
  // use readline to get pet name + etc
  rl.question('name?', (name) => {
    pet = { name }
    
    // callback is called after question completes because
    //  it is included inside of the question callback
    callback(pet)
  })
  
  // if we called callback here it would happen immediately
  //  (before question completes) and we would not have
  //  access to the response to the question
}

module.exports = start