Read validate-forms.js from assets.

site/static/docs/5.0/assets/js/validate-forms.js

@@ -0,0 +1,20 @@
+// Example starter JavaScript for disabling form submissions if there are invalid fields
+(function () {
+  'use strict'
+
+  // Fetch all the forms we want to apply custom Bootstrap validation styles to
+  var forms = document.querySelectorAll('.needs-validation')
+
+  // Loop over them and prevent submission
+  Array.prototype.slice.call(forms)
+    .forEach(function (form) {
+      form.addEventListener('submit', function (event) {
+        if (!form.checkValidity()) {
+          event.preventDefault()
+          event.stopPropagation()
+        }
+
+        form.classList.add('was-validated')
+      }, false)
+    })
+})()