zalgo.js

function synchCallback(opts, cb) {
  if (!opts.someImportantProperty) return cb(new Error('Missing important property'))
  doSomeStuff(opts, function(err, result) {
    console.log(err, result)
    // ..
    cb(err, result)
  })
}

synchCallback({}, function(err, results) {
  // this callback will be called here, before '*'
})

// '*'
console.log('this will be called after the callback above')