promiseInstance.finally(onFinally)

function checkMail() {
  return new Promise((resolve, reject) => {
    if (Math.random() > 0.5) {
      resolve('Mail has arrived')
    } else {
      reject(new Error('Failed to arrive'))
    }
  })
}
 
checkMail()
  .then((mail) => {
    console.log(mail)
  })
  .catch((err) => {
    console.error(err)
  })
  .finally(() => {
    console.log('Experiment completed')
  })