promise-demo.js

promiseReturningService
  .getUserData()
  .then((userData) => {
    // this only runs if there is no error getting user data
    return userData.userName;
  })
  .then((userName) => {
    // this only runs if there is no error in all previous calls
    console.log(userName);
  })
  .catch((error) => {
    // this only runs if there is an error
    console.log(error.message);
  })
  .finally(() => {
    console.log('This always runs');
  });

promise-internal.js

function getUserData() {
  return {
    then: function (onSuccess, onFailure) {
      onSuccess(userData);
      return this;
    },
    catch: function (onFailure) {
      const promiseError = new Error('An error occurred');
      onFailure(promiseError);
      return this;
    },
    finally: function (onComplete) {
      onComplete();
      return this;
    }
  };
}