Handling Rejection Errors in Javascript Promises

Question

I have a Node.js App and is using a function that returns a Promise, but it seems in some conditions it gives:

Unhandled rejection Error: getaddrinfo ENOTFOUND
    at errnoException (dns.js:37:11)
    at Object.onanswer [as oncomplete] (dns.js:124:16)

And my server crashes.

Here is a simplifed code that does the crash if wifi is turned off:

checkip.getExternalIp().then(function (ip) {
  console.log("External IP = "+ip);
});

Is there a way to handle something like this?

Answer 1

you have two options. You can slap a .catch call on to the end of the promise, or you can provide a second callback to .then which runs in the failed case:

checkip.getExternalIp().then(function (ip) {
  console.log("External IP = "+ip);
}).catch(function(err) {
  // handle error here
});

or

checkip.getExternalIp().then(function (ip) {
  console.log("External IP = "+ip);
}, function(err) {
  // handle error here
});

Answer 2

ES2015 promises follow the A+ spec . Thus, any promise has the possibility of generating an error that can be caught. So, to trap the error you are receiving simply supply a catch handler:

checkip.getExternalIp()
  .then(then_handler)
  .catch(catch_handler);

More complete examples are in the documentation .