"Illegal choice 0 in Book element" when switching the book outline field from anything to "- None - "

I feel like this may be a duplicate but I don't have time to search at the moment.

I don't see a duplicate. This is still a bug. If the page is already in a book you don't get an error. But if it's not in a book, you can't use "Create a new book" to make one.

The bug is centered in \Drupal\book\BookManager::addFormElements. When Ajax reevaluates the form element, the "0" value is not added to the options.

    if (!$node->book['bid']) {
      // The node is not currently in the hierarchy.
      $options = [0 => $this->t('- None -')] + $options;
    }

    // Add a drop-down to select the destination book.
    $form['book']['bid'] = [
      '#type' => 'select',
      '#title' => $this->t('Book'),
      '#default_value' => $node->book['bid'],
      '#options' => $options,
      '#access' => (bool) $options,
      '#description' => $this->t('Your page will be a part of the selected book.'),
      '#weight' => -5,
      '#attributes' => ['class' => ['book-title-select']],
      '#ajax' => [
        'callback' => 'book_form_update',
        'wrapper' => 'edit-book-plid-wrapper',
        'effect' => 'fade',
        'speed' => 'fast',
      ],
    ];

(edited - fixed typos)

This is happening becuase $form['book']['bid']['options'] changes depending on the choice.

Thank you @vomiand!

The identical operator could be used here:

$nid == 'new'

We should be able to add a front-end test for this.

Updated comparison operator.

@vomiand #8patch works without issue and warning removed.

Lets add some tests for this.

Patch#8 working fine and also the test cases are good and succeed.

Let's wait for others to review and we can move this to RTBC.

I applied the patch and worked as expected, I think we can move this to RTBC.

Fixing the file list so that the latest patch is shown, and updating issue credits. Thanks for working on this issue!

+++ b/core/modules/book/tests/src/FunctionalJavascript/BookJavascriptTest.php
@@ -157,4 +157,23 @@ protected function assertOrderInPage(array $items) {
+    $book_select->setValue('new');
+    $session->wait(10000);
+    $book_select->setValue(0);
+    $session->wait(10000);

Rather than waiting for time to pass here we should be asserting something.

After selecting 'new' we should wait for the text This will be the top-level page in this book. and then test that No book selected. is not there.

And after selecting '0' we should wait for the text No book selected. and test that This will be the top-level page in this book. is no longer there.

Given this is over a year old I made the changes recommended by #19 myself.

Forgot to upload an interdiff.

Patch #20 is working fine in Drupal - 8.8.6, thanks @alexpott.

Patch applied successfully to 9.2.0-dev

Tests passing. RTBC

Committed/pushed to 9.2.x and cherry-picked back through to 8.9.x, thanks!