Prevent invalid characters in number input field

js/number-input.js

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2025 SYSTOPIA GmbH
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Restricts input of number fields to digits, decimal separator, and minus.
+ */
+(function (Drupal, once) {
+  Drupal.behaviors.json_forms_numberInput = {
+    attach: function (context, settings) {
+      // Decimal separator of browser.
+      const decimalSeparator = Intl.NumberFormat()
+        .formatToParts(1.1)
+        .find(part => part.type === 'decimal')
+        .value;
+      const digits = '0123456789';
+
+      function getElementLang(element) {
+        if (element.lang) {
+          return element.lang;
+        }
+
+        return element.parentElement ? getElementLang(element.parentElement) : null;
+      }
+
+      once('json-forms-number-input', 'input[type="number"]', context).forEach((element) => {
+        let decimalSeparators = decimalSeparator;
+        const lang = getElementLang(element);
+        if (lang) {
+          // Decimal separator of element's language might be different from
+          // the browser's separator. We allow both in that case. It depends on
+          // the browser which one is preferred, i.e. the one that is used when
+          // pressing the buttons to change a number.
+          decimalSeparators += Intl.NumberFormat(lang)
+            .formatToParts(1.1)
+            .find(part => part.type === 'decimal')
+            .value;
+        }
+
+        element.addEventListener('keydown', function (event) {
+          if (event.key === 'Backspace' || '0123456789'.indexOf(event.key) !== -1) {
+            return;
+          }
+
+          if ('-' === event.key) {
+            if (element.value.indexOf('-') === -1 && (element.min < 0 || element.min === '' || element.min == null)) {
+              return;
+            }
+          }
+          else if (decimalSeparators.indexOf(event.key) !== -1) {
+            if (element.value.indexOf('.') === -1 && (element.step === 'any' || element.step < 1)) {
+              return;
+            }
+          }
+
+          event.preventDefault();
+        });
+      });
+    }
+  };
+
+})(Drupal, once);

json_forms.libraries.yml

@@ -16,6 +16,14 @@ disable_buttons_on_ajax:
   dependencies:
     - core/jquery
 
+number_input:
+  version: 0.1.0
+  js:
+    js/number-input.js: {}
+  dependencies:
+    - core/drupal
+    - core/once
+
 vertical_tabs:
   version: 0.1.0
   js:

src/Form/Control/NumberArrayFactory.php

@@ -50,6 +50,7 @@ public function createFormArray(
       '#type' => 'number',
       '#value_callback' => NumberValueCallback::class . '::convert',
       '#_type' => $definition->getType(),
+      '#attached' => ['library' => ['json_forms/number_input']],
     ] + BasicFormPropertiesFactory::createFieldProperties($definition, $formState);
 
     if (NULL !== $definition->getExclusiveMinimum()) {