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API Handling failed responses

You can configure how Unpoly handles failed server responses.

Some example for failed responses are:

  • A form submission fails due to invalid user input. The server re-renders the form with validation errors.
  • The server renders unexpected content, like a login screen or a maintenance page.
  • The server-side app crashes with an HTTP 500 server error.
  • The request is aborted by a second request targeting the same fragment.

Rendering failed responses differently

You may pass different render options for server responses with an error code, which we will call failed responses below. Any HTTP status other than 2xx or 304 is considered a failed response.

A common use case for this is form submissions, where a successful response should display a follow-up screen, but a failed response should re-render the form with validation errors. A good HTTP status code for an invalid form submission is 422 (Unprocessable Entity).

To use a different render option for a failed server response, prefix the option with fail:

  url: '/action',
  method: 'post',
  target: '.content',            // when submission succeeds update '.content'
  failTarget: 'form',            // when submission fails update the form
  scroll: 'auto',                // when submission succeeds use default scroll behavior
  failScroll: '.errors',         // when submission fails scroll to the error messages
  onRendered: () => { ... },     // when submission succeeds run this callback
  onFailRendered: () => { ... }, // when submission fails run this other callback

Options that are used before the request is made (like { url, method, confirm }) do not have a fail-prefixed variant. Some options (like { history, fallback }) are used for both successful and failed responses, but may be overriden with a fail-prefixed variant (e.g. { history: true, failHistory: false }. Options related to layers, scrolling or focus are never shared.

Note that event handlers still begin with on for failed responses, so { onRendered } becomes { onFailRendered }.

When using Unpoly's HTML attributes with links or forms you may infix an attribute with fail:

<form method="post" action="/action"
  up-target=".content"      <!-- when submission succeeds update '.content' -->
  up-fail-target="form"     <!-- when submission fails update the form -->
  up-scroll="auto"          <!-- when submission succeeds use default scroll behavior -->
  up-fail-scroll=".errors"> <!-- when submission fails scroll to the error messages -->

Also see validating forms.

Ignoring HTTP error codes

With { fail: false } or [up-fail=false] Unpoly will always consider the response to be successful, even with a HTTP 4xx or 5xx status code.

Customizing failure detection

By default any HTTP 2xx or 304) status code will be considered successful, and any other status code will be considered failed. This behavior can be customized. For instance, you can fail a response if it contains a given header or body text.

The following configuration will fail all responses with an X-Unauthorized header:

let badStatus = up.network.config.fail
up.network.config.fail = (response) => badStatus(response) || response.header('X-Unauthorized')

You can also decide to fail a response in an up:fragment:loaded listener:

up.on('up:fragment:loaded', function(event) {
  if (event.response.header('X-Unauthorized')) {
    event.renderOptions.fail = true

Local content cannot fail

When the updated fragment content is not requested from a { url }, but rather passed as an HTML string ({ document, fragment, content }), the update is always considered successful.

Handling unexpected content

A server might sometimes respond with unexpected content, like a maintenance page or a login form. In these cases a specific target selector may not match in the server response.

With { fallback: true } or [up-fallback=true] Unpoly will try to match a main target before giving up. This will show the server content in the application's main content area, which is often preferred to a link appearing dead. Fallback targets are enabled by default when navigating.

To configure a custom handling of unexpected content, use the up:fragment:loaded event.

Handling fatal network errors

When a request encounters fatal error like a timeout or loss of network connectivity, Unpoly will emit up:request:offline and not render.

Because there never was a server response, up:fragment:loaded will not be emitted in this case.

Handling aborted requests

When a request was aborted, Unpoly will emit up:request:aborted and not render.

A promise for an aborted request will reject with an up.AbortError.

By default Unpoly will abort a request when a second request targets the same fragment.

Detecting a failed response programmatically

Rendering functions like up.render(), up.follow() or up.submit() return a promise that rejects when the server sends a failed response, or when there is another error.

See render error handling example.