Handling error in Promises

Jeff Morrison lbljeffmo at gmail.com
Tue Jan 13 08:27:12 PST 2015

You might try reading through some of the previous threads that talk 
about the trickiness of surfacing promise errors in the general case.

Unfortunately these threads usually devolve into an endless thread of 
discussion and debate that become next to impossible to actually read 
through later without several hours of spare time and a bottle of 
whiskey... But here's a recent one that's not *too* long and kind of 
talks about a couple of ideas:

https://esdiscuss.org/topic/async-await-improvements (long, but talks 
about a couple potential/proposed solutions)


On 1/13/15 7:44 AM, Marius Gundersen wrote:
> A `promise.done()` method that throws if it receives a rejected 
> promise has been discussed, but the consensus seems to be that 
> browsers instead should report on rejected unhandled promises that are 
> garbage collected. This is already implemented in Firefox (at least in 
> the DevTools edition), where the following code will end up with an 
> error in the console:
> ```js
> var p = new Promise(function(){throw new Error("oh noes");})
> p = null;
> ```
> Since there is no way to handle the thrown error outside the promise 
> (the exception is thrown after the function returns) there isn't any 
> reason for the exception to travel up the stack. Either the exception 
> must be handled as a rejected promise or it must be handled in 
> something like `window.onerror`.
> Marius Gundersen
> On Tue, Jan 13, 2015 at 12:35 PM, Boopathi Rajaa 
> <legend.raju at gmail.com <mailto:legend.raju at gmail.com>> wrote:
>     ```
>     Promise
>       .resolve()
>       .then(function(){
>         throw new Error('asdf');
>       });
>     ```
>     Bluebird: (errors thrown - Good)
>     http://jsbin.com/kahuzi/1/edit?html,js,console
>     native ES6:(errors not thrown)
>     http://jsbin.com/qivobibowa/3/edit?html,js,console
>     Shouldn't all Uncaught errors be thrown, instead of catching it
>     inside the promise only ? For example, If I use Promise within a
>     promise, the uncaught error in the inner Promise will never be
>     exposed to the outside world and there is no way to bubble it up
>     to the outside blocks.
>     `.catch(function(err) { throw err; }) `
>     will also be caught and nothing would be thrown.
>     - Boopathi
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