Let’s start here:
Promise.resolve( 'hi' ).then( showHappy ).catch( showError )
This is a super common form you see with Promises, but is has a subtle problem. What happens if your
then callback throws?
catch caught the error! Because of how promises chain, this is expected, but often surprising. If your
catch is written to expect a certain kind of error – maybe from a network request – it too could now fail.
If you want to be sure that the rejected promise is from the original promise, you have two options: use the second parameter to
then, or write the
These methods have their own issues though. If the success handler really may throw, you may need to add a second
catch to the end to handle that case.
And if you put the
catch first, if that handler returns a non-Error, then following
then will run:
That’s rarely what you want, but can be handy for cases where you want to transform an API error into a blank result.
Next time, how this all changes with
async / await.