javascript - Difference between finally() and then() in ES2018

Question

Before ES2018, I used to nest an extra then at the end of the promise chain whenever I had to perform any clean up logic that I'd otherwise duplicate in then and catch above, e.g.

new Promise(
  (res, rej) => setTimeout(() => rej({}), 1000)
).then(
  res => console.log(res)
).catch(
  err => console.error(err)
).then(
  () => console.log('Finally')
)

Answer 1

A then callback doesn't execute when the promise is rejected - and that can happen even for a promise returned by a catch invocation: when its callback throws or returns a rejected promise. err => console.error(err) is probably not going to do that, but you never know.

Similarly, I would recommend to favour .then(…, …) over .then(…).catch(…) if you only want to handle errors from the original promise, not from the then callback. I'd write

promise.then(console.log, console.error).finally(() => console.log('Finally'));

The other more or less obvious differences are in the signature: the finally callback doesn't receive any arguments, and the promise that p.finally() returns will fulfill/reject with the same result as p (unless there's an exception or returned rejection in the callback).