Demonstration of UnhandledPromiseRejectionWarning

unhandled.js

// Do a thing poorly and asynchronously
async function doAThingBadly() {
  throw new Error('It broke')
}

// node uncaught.js
// Try to await a thing. It will throw, but it seems like the Error
// should bubble up to be caught in the higher-up try-catch, right? Right?
async function doAThing() {
  const thing = await doAThingBadly()
  
  return thing
}

// node uncaught.js --try
// Try to await a thing. It will throw, but we can catch it here and fix it.
// Whatever we return here will be the Promise's resolved value
async function doAThingWithTry() {
  try {
    const thing = await doAThingBadly()
    
    return thing
  }
  catch (error) {
    console.error('* Caught inside doAThingWithTry():', error.message)
    return 'it threw, but we try-caught it'
  }
}

// node uncaught.js --catch
// Try to await a thing. It will throw, but we can .catch() it here or
// further up the call tree.
async function doAThingWithDotCatch() {
  const thing = await doAThingBadly().catch(error => {
    console.error('* caught in .catch()', error.message)
    // from here, you can change the value that await will see
    return 'it threw, but we dot-caught it'
  })

  return thing
}


// Decide what to do, based on the Node commandline
let thingsFunction = doAThing
let be_safe = 'no'

if (process.argv.includes('--try')) {
  be_safe = 'try-catch'
  thingsFunction = doAThingWithTry
}
else if (process.argv.includes('--catch')) {
  be_safe = 'dot-catch'
  thingsFunction = doAThingWithDotCatch
}


console.log('About to do a thing with', be_safe, 'safety')

try {
  thingsFunction().then(result => {
    console.log('We did a thing!', `result = ${result}`)
    console.log('(but there was no exception into the main thread!)')
  })
  // You need theis .catch(), in case there is an uncaught rejection 
  // deeper inside the call tree: an exception in doAThingBadly() can be
  // caught in an immediate try-catch, but if it bubbles up to here then
  // it becomes an Unhandled Rejection and will complain (or, soon, exit
  // with a failure code):
  //
  // .catch(error => {
  //   console.log('* Caught in main.catch:', error.message)
  //   process.exit(1)
  // })
}
catch (error) {
  console.log('* Caught in main:', error.message)
  process.exit(2)
}

console.log('End of main script; now we\'re async...')