Using fetchApi I am issuing a POST request to an endpoint and catching errors like this:

fetch(new Request(url, {
  method: 'POST',
  headers: { ...defaultHeaders, ...headers } ,
  credentials: 'include',
  body: JSON.stringify(body)
})).then(
  // handle correct reponse
).catch(
  if (err.status === 401) {
       // handle error nicely
    } else if (err.status === 429) {
       // handle error differently
    } else {
      // handle error yet another way
    }
);

While 401 error is handled as expected, when the endpoint responds with 429 the code

else if (err.status === 429) {
       // handle error differently
    } 

Is never executed.

EDIT1: the entire catch is never reached in case of 429

Are 401 and 429 status codes handled differently by javascript/browsers? How Can I catch 429 error and handle it my way?

Using fetchApi I am issuing a POST request to an endpoint and catching errors like this:

fetch(new Request(url, {
  method: 'POST',
  headers: { ...defaultHeaders, ...headers } ,
  credentials: 'include',
  body: JSON.stringify(body)
})).then(
  // handle correct reponse
).catch(
  if (err.status === 401) {
       // handle error nicely
    } else if (err.status === 429) {
       // handle error differently
    } else {
      // handle error yet another way
    }
);

While 401 error is handled as expected, when the endpoint responds with 429 the code

else if (err.status === 429) {
       // handle error differently
    } 

Is never executed.

EDIT1: the entire catch is never reached in case of 429

Are 401 and 429 status codes handled differently by javascript/browsers? How Can I catch 429 error and handle it my way?

• javascript
• http
• fetch-api

• 1 Is the catch executed at all? Then have you debugged whether err.status is really exactly 429? – deceze ♦ Commented Sep 15, 2017 at 13:13
• The catch is executed in case of 401 status code but is not executed at all in case of 429. I know it is 429 indeed because the error is logged in the JS console in the browser. – Piotr Zakrzewski Commented Sep 15, 2017 at 13:22
• 1 OK, so the problem is not that the else if isn't matching, it's that fetch doesn't reject the promise, right? – deceze ♦ Commented Sep 15, 2017 at 13:24
• 1 @PiotrZakrzewski Looks like fetch API does not reject when response status is 429. Instead, you may need to handle it in then logic. – shaochuancs Commented Sep 15, 2017 at 13:30
• 1 Playing around with httpstat.us, I can't get any status code to reject. Poking through the spec it appears that the promise is only rejected if the response's type is "error", but in these example it's always "cors". Maybe these observations will help… – deceze ♦ Commented Sep 15, 2017 at 13:42

1 Answer 1

Looks mostly like wrong expectations. The promise will only be rejected (and hence catch invoked) when the response's type is "error", which only seems to be the case for very specific, limited cases.

Per MDN, the fetch() API only rejects a promise when a “network error is encountered, although this usually means permissions issues or similar.” Basically fetch() will only reject a promise if the user is offline, or some unlikely networking error occurs, such a DNS lookup failure.

In other words, a request with a 429 response is still a successfully executed request.

The good is news is fetch provides a simple ok flag that indicates whether an HTTP response’s status code is in the successful range or not.

fetch("http://httpstat.us/500")
    .then(function(response) {
        if (!response.ok) {
            throw Error(response.statusText);
        }
    })

https://www.tjvantoll./2015/09/13/fetch-and-errors/