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Promise: then versus catch

Are these code fragments equal? In other words, do they behave the same way in any circumstances, for any handler functions?

promise.then(f1, f2);



The short answer is: no, they are not the equal:

The difference is that if an error happens in f1, then it is handled by .catch here:


…But not here:

  .then(f1, f2);

That’s because an error is passed down the chain, and in the second code piece there’s no chain below f1.

In other words, .then passes results/errors to the next .then/catch. So in the first example, there’s a catch below, and in the second one – there isn’t, so the error is unhandled.