In a Promise, what's the difference between using catch and the 2nd argument of then? [duplicate]

Answer 1

Besides .catch(fn) being a shortcut for .then(null, fn), the difference in your examples is that

funcThatReturnsAPromise()
  .then(() => { /* success */ })
  .catch(() => { /* fail */ });

// is equivalent to

const p1 = funcThatReturnsAPromise()
const p2 = p1.then(() => { /* success */ })
const p3 = p2.catch(() => { /* 
   executed if p1 is rejected
   executed if p2 is rejected 
*/ })

While the second one is

funcThatReturnsAPromise()
  .then(() => { /* success */ }, () => { /* fail */ });

// equivalent to

const p1 = funcThatReturnsAPromise()
const p2 = p1.then(
  () => { /* success */ },
  () => { /*
     executed if p1 is rejected
     (p2 will be actually resolved by the result of this function only when p1 is rejected)
  */ }
);