303 See Other

August 25, 2019 • 2 min read

Last week I learned a few things about how redirects and 303 status codes are handled in the browser.

In the app I’m building the following happens: Once a session expires any requests to the server are blocked and redirected to the url of the login (html) webpage. This also happens for any fetch requests (ajax calls) our single page app is making. My job this week was to handle these redirects gracefully and show users some Hello, your session has expired please login again page. This turned out to be more challenging than I anticipated.

I probably should have known, but 303 status codes are not visible on the client (i.e. in my JavaScript code). So my plan to just check for any 303’s and redirect based on that, didn’t pan out. The only errors I was seeing, were CORS errors since the login page is outside our domain.

My next strategy was to use the redirect: ‘error’ option that the fetch API provides, like so:

fetch('https://httpstat.us/303', { redirect: “error” })
    .then(console.log)
    .catch(console.error);

In Chrome this request fails with a TypeError: Failed to fetch. Could I use this error to detect expired sessions? In Firefox I got the same error message. Hurrah!?

However, on Safari I got TypeError: Not allowed to follow a redirection while loading https://httpstat.us/303. That’s actually more informative, but in general I got a bad feeling about this approach: The error messages were not consistent across browsers and the Failed to fetch on Chrome could be any kind of failure. I could not be sure these failures were related to redirects (and thus expired user sessions).

My final approach was based on using the manual redirect Enum. That gave me a little bit more information on modern browsers:

fetch('https://httpstat.us/303', { redirect: 'manual' })
  .then(console.log)
  .catch(console.error)

This request results in a Response object with a type: “opaqueredirect” property on all modern browsers. This was something, but of course it does not work on older browsers like IE11 (for which our app relies on a fetch polyfill).

So even this final approach did not give me a reliable way to detect expired sessions. As a temporary solution it kinda works. On IE11 we’re now showing users a general error message on failed requests that are missing a status code. These messages offer users a “Try to login again” link.

In the meantime other engineers are working on a solution to let the client know directly that a session has expired. If I had to choose a solution I would go for a response with status 401 (authentication is possible but has failed or was not yet been provided.). And in the response body I would expect the reason. In json/text ‘cause that’s what I’m requesting in my headers as the Content-Type. Then I can absolutely know for sure what’s going on.

(Note: There’s also the redirected property on the Response object, however this is still an experimental feature that’s not available in all browsers, Safari and Internet Explorer being notable exceptions for my purposes. See the MDN page for Response)

Got any questions, found a mistake? You can find me on Twitter as @vnglst. You can also discuss the article on TwitterSuggest changes on Gitlab


Koen van Gilst

Koen van Gilst

Personal blog by Koen van Gilst. JavaScript developer, M.A. in Philosophy, former translator.