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).catch(f2);
Versus:
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:
promise
.then(f1)
.catch(f2);
…But not here:
promise
.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.