An example "always" behavior for ES6 promises. This only works if you do not create / return intermediate promises.

promise.js

// A thing I want to do
// This flow only involves **one** promise, for example an ajax call
// None of the subsequent `then` or `catch` calls, return new promises.



const explode = false;
const promise = new Promise((resolve, reject) => {
  if (explode) {
    reject();
  } else {
    resolve();
  }
}).then(() => {
  console.log('Thing is done. Do followup task.');
  return 'my argument';
}).then((arg1) => {
  console.log('Thing is done. Do followup task 2. argument: ' + arg1);
}).catch(() => {
  console.log('Thing failed');
}).then(() => {
  console.log('Always do this');
});
// => "Thing is done. Do followup task."
// => "Thing is done. Do followup task 2. argument: my argument"
// => "Always do this"



// Set explode to true
const explode = true;
const promise = new Promise((resolve, reject) => {
  if (explode) {
    reject();
  } else {
    resolve();
  }
}).then(() => {
  console.log('Thing is done. Do followup task.');
  return 'my argument';
}).then((arg1) => {
  console.log('Thing is done. Do followup task 2. argument: ' + arg1);
}).catch(() => {
  console.log('Thing failed');
}).then(() => {
  console.log('Always do this');
});
// => "Thing failed"
// => "Always do this"