Using promise.then(callback, callback) misses errors
One traditional example of using promises is
There exists one important caveat in this approach. Can you spot it?
Exception in success handler goes unnoticed
If the success handler throws exception, then it will not be caught and handled in the rejection handler as one would think.
An improved version would be to separate the error handler as its own step. Then chain that only after the success handler. This way exception in the success handler will be propagated to the correct place.
A real world example
This becomes important in longer promise chains. For example if
renderResultPage throws an error, it will not end up in
handleError as you might expect:
it's better to use
catch() to avoid this
and catch all errors.
Mocha test case
Test case available at