Best practices of attaching a promise after rejection

Question

Say we have functions A and B that return promises. B should be called only in case A rejects, and I need to log both success and error results of both promises. First, I came up with this piece of code:

const A = () => Promise.reject(true);
const B = () => Promise.resolve(true);

A()
  .then(() => {
    console.log('A success');
  })
  .catch(() => {
    console.log('A error');
    return B();
  })
  .then(() => {
    console.log('B success');
  })
  .catch(() => {
    console.log('B error');
  });

It works fine in case A rejects. In case A resolves, however, I'm getting one additional B success message, which is perfectly expected but which I do not need in this case.

One possible solution I came up with is attaching the .then() for B() right next to the B() call.

const A = () => Promise.reject(true);
const B = () => Promise.resolve(true);

A()
  .then(() => {
    console.log('A success');
  })
  .catch(() => {
    console.log('A error');

    return B().then(() => {
      console.log('B success');
    });
  })
  .catch(() => {
    console.log('B error');
  });

This of course solves my problem but it feels like more of a hack than a solution. I believe there are better approaches and practices for composing this kind of promise chains.

Your second approach looks perfectly fine to me. If you want it to look cleaner you could switch some of the `.then`s with `await`s. If the chain becomes more complicated, you can extract into functions, eg `handleAError` — CertainPerformance, Jul 10 '22 at 09:33
Not a hack, that's basically how you do it if you use explicit promise callbacks. (Side note: You mean "if A is fulfilled," not "if A resolves" when contrasting with A rejecting. I write about promise terminology here: https://thenewtoys.dev/blog/2021/02/08/lets-talk-about-how-to-talk-about-promises/) — T.J. Crowder, Jul 10 '22 at 09:34

score 1 · Answer 1 · answered Jul 10 '22 at 15:14

This is quite fine. However, notice that your catch handlers also run if any of the previous then handlers did cause an error, not only if A() or B() errored. I therefore recommend using .then(…, …) instead of .then(…).catch(…):

A().then(() => {
  console.log('A success');
}, e => {
  console.log('A error', e);
  return B().then(() => {
    console.log('B success');
  }, e => {
    console.log('B error', e);
  });
});

Best practices of attaching a promise after rejection

1 Answers1