Interactively guide users for selecting a valid username

frontend/src/entrypoints/templates.ts

@@ -0,0 +1,98 @@
+/* Copyright 2025 New Vector Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+// This script includes some optional javascript used in the server-side
+// generated templates which enhances the user experience if loaded.
+//
+// Ideally later on we could find a way to hydrate full React components instead
+// of doing this, as this can very quickly get out of hands.
+
+const VALID_USERNAME_RE = /^\s*([a-z0-9.=_/-]+|@[a-z0-9.=_/-]+(:.*)?)\s*$/g;
+
+/** Grab the nearest error message inserted by the templates by error kind and code */
+function grabErrorMessage(
+  parentElement: HTMLElement | null,
+  kind: string,
+  code: string,
+): HTMLElement | null {
+  if (!parentElement) return null;
+  const matching = parentElement.querySelectorAll<HTMLElement>(
+    `[data-error-kind=${kind}][data-error-code=${code}]`,
+  );
+  // We potentially have duplicate error messages coming from the templates, one
+  // hidden and one visible
+  let el: HTMLElement | null = null;
+  for (const element of matching) {
+    // In case we're finding a non-hidden element, we prioritise that one
+    if (!element.classList.contains("hidden")) return element;
+    // Else it will be the last element in the list
+    el = element;
+  }
+  return el;
+}
+
+/**
+ * This patches a username input element to lowercase on input and trim on blur
+ *
+ * @param inputElement The input element to patch
+ */
+function patchUsernameInput(inputElement: HTMLInputElement) {
+  // Exclude readonly/disabled inputs
+  if (inputElement.readOnly || inputElement.disabled) return;
+
+  const labelElement = inputElement.parentElement?.querySelector("label");
+  // This is the list of elements which needs to have the data-invalid attribute
+  // set/unset
+  const fieldElements: HTMLElement[] = [inputElement];
+  if (labelElement) fieldElements.push(labelElement);
+
+  // Grab the translated 'invalid username' message from the DOM
+  // TODO: we could expand this to other validation messages, but this is the
+  // most important one for now
+  const invalidUsernameMessage = grabErrorMessage(
+    inputElement.parentElement,
+    "policy",
+    "username-invalid-chars",
+  );
+  if (!invalidUsernameMessage) {
+    console.warn(
+      "Could not find the error message in the DOM for username validation",
+      inputElement,
+    );
+  }
+
+  inputElement.addEventListener("input", function () {
+    // Simply lowercase things automatically, as this is not too disruptive
+    inputElement.value = inputElement.value.toLocaleLowerCase();
+
+    const match = inputElement.value.match(VALID_USERNAME_RE);
+    if (!inputElement.value.trim() || match !== null) {
+      // Remove the data-invalid attribute from all elements
+      for (const el of fieldElements) el.removeAttribute("data-invalid");
+
+      // Hide the error message
+      invalidUsernameMessage?.classList.add("hidden");
+    } else {
+      // Set the data-invalid attribute on all elements
+      for (const el of fieldElements) el.setAttribute("data-invalid", "");
+
+      // Show the error message
+      invalidUsernameMessage?.classList.remove("hidden");
+    }
+  });
+
+  // Sneakily trim the input on blur
+  inputElement.addEventListener("blur", function () {
+    inputElement.value = inputElement.value.trim();
+  });
+}
+
+// Look for username inputs on the page and patch them
+for (const element of document.querySelectorAll<HTMLInputElement>(
+  "input[data-choose-username]",
+)) {
+  patchUsernameInput(element);
+}

templates/base.html

@@ -25,6 +25,7 @@ Please see LICENSE files in the repository root for full details.
     <meta name="viewport" content="width=device-width, initial-scale=1">
     {{ include_asset('src/entrypoints/shared.css') | indent(4) | safe }}
     {{ include_asset('src/entrypoints/templates.css') | indent(4) | safe }}
+    {{ include_asset('src/entrypoints/templates.ts') | indent(4) | safe }}
     {{ captcha.head() }}
   </head>
   <body>

templates/components/field.html

@@ -19,6 +19,46 @@ Please see LICENSE files in the repository root for full details.
   {%- if value %} value="{{ value }}" {% endif %}
 {%- endmacro %}
 
+{% macro error(error, hidden=false) %}
+  <div class="cpd-form-message cpd-form-error-message
+    {%- if hidden %} hidden{% endif %}" data-error-kind="{{ error.kind }}"
+    {%- if error.code %}data-error-code="{{ error.code }}"{% endif %}>
+    {% if error.kind == "required" %}
+      {{ _("mas.errors.field_required") }}
+    {% elif error.kind == "exists" and field.name == "username" %}
+      {{ _("mas.errors.username_taken") }}
+    {% elif error.kind == "policy" %}
+      {% if error.code == "username-too-short" %}
+        {{ _("mas.errors.username_too_short") }}
+      {% elif error.code == "username-too-long" %}
+        {{ _("mas.errors.username_too_long") }}
+      {% elif error.code == "username-invalid-chars" %}
+        {{ _("mas.errors.username_invalid_chars") }}
+      {% elif error.code == "username-all-numeric" %}
+        {{ _("mas.errors.username_all_numeric") }}
+      {% elif error.code == "username-banned" %}
+        {{ _("mas.errors.username_banned") }}
+      {% elif error.code == "username-not-allowed" %}
+        {{ _("mas.errors.username_not_allowed") }}
+      {% elif error.code == "email-domain-not-allowed" %}
+        {{ _("mas.errors.email_domain_not_allowed") }}
+      {% elif error.code == "email-domain-banned" %}
+        {{ _("mas.errors.email_domain_banned") }}
+      {% elif error.code == "email-not-allowed" %}
+        {{ _("mas.errors.email_not_allowed") }}
+      {% elif error.code == "email-banned" %}
+        {{ _("mas.errors.email_banned") }}
+      {% else %}
+        {{ _("mas.errors.denied_policy", policy=error.message) }}
+      {% endif %}
+    {% elif error.kind == "password_mismatch" %}
+      {{ _("mas.errors.password_mismatch") }}
+    {% else %}
+      {{ error.kind }}
+    {% endif %}
+  </div>
+{% endmacro %}
+
 {% macro field(label, name, form_state=false, class="", inline=false) %}
   {% set field_id = new_id() %}
   {% if not form_state %}
@@ -55,41 +95,7 @@ Please see LICENSE files in the repository root for full details.
     {% if field.errors is not empty %}
       {% for error in field.errors %}
         {% if error.kind != "unspecified" %}
-          <div class="cpd-form-message cpd-form-error-message">
-            {% if error.kind == "required" %}
-              {{ _("mas.errors.field_required") }}
-            {% elif error.kind == "exists" and field.name == "username" %}
-              {{ _("mas.errors.username_taken") }}
-            {% elif error.kind == "policy" %}
-              {% if error.code == "username-too-short" %}
-                {{ _("mas.errors.username_too_short") }}
-              {% elif error.code == "username-too-long" %}
-                {{ _("mas.errors.username_too_long") }}
-              {% elif error.code == "username-invalid-chars" %}
-                {{ _("mas.errors.username_invalid_chars") }}
-              {% elif error.code == "username-all-numeric" %}
-                {{ _("mas.errors.username_all_numeric") }}
-              {% elif error.code == "username-banned" %}
-                {{ _("mas.errors.username_banned") }}
-              {% elif error.code == "username-not-allowed" %}
-                {{ _("mas.errors.username_not_allowed") }}
-              {% elif error.code == "email-domain-not-allowed" %}
-                {{ _("mas.errors.email_domain_not_allowed") }}
-              {% elif error.code == "email-domain-banned" %}
-                {{ _("mas.errors.email_domain_banned") }}
-              {% elif error.code == "email-not-allowed" %}
-                {{ _("mas.errors.email_not_allowed") }}
-              {% elif error.code == "email-banned" %}
-                {{ _("mas.errors.email_banned") }}
-              {% else %}
-                {{ _("mas.errors.denied_policy", policy=error.message) }}
-              {% endif %}
-            {% elif error.kind == "password_mismatch" %}
-              {{ _("mas.errors.password_mismatch") }}
-            {% else %}
-              {{ error.kind }}
-            {% endif %}
-          </div>
+          {{ error(error) }}
         {% endif %}
       {% endfor %}
     {% endif %}

templates/pages/register/index.html

@@ -28,7 +28,7 @@ Please see LICENSE files in the repository root for full details.
 
     {% if features.password_registration %}
       {% call(f) field.field(label=_("common.username"), name="username", form_state=form) %}
-        <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="off" />
+        <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="off" data-choose-username />
         <div class="cpd-form-message cpd-form-help-message" id="{{ f.id }}-help">
           @username:{{ branding.server_name }}
         </div>

templates/pages/register/password.html

@@ -32,7 +32,8 @@ Please see LICENSE files in the repository root for full details.
     <input type="hidden" name="csrf" value="{{ csrf_token }}" />
 
     {% call(f) field.field(label=_("common.username"), name="username", form_state=form) %}
-      <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="none" required />
+      <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="none" required data-choose-username />
+      {{ field.error(error={"kind": "policy", "code": "username-invalid-chars"}, hidden=true) }}
     {% endcall %}
 
     {% if features.password_registration_email_required %}

templates/pages/upstream_oauth2/do_register.html

@@ -99,7 +99,8 @@ Please see LICENSE files in the repository root for full details.
       {% endcall %}
     {% else %}
       {% call(f) field.field(label=_("common.username"), name="username", form_state=form_state) %}
-        <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="none" value="{{ imported_localpart or '' }}" aria-describedby="{{ f.id }}-help" />
+        <input {{ field.attributes(f) }} class="cpd-text-control" type="text" autocomplete="username" autocorrect="off" autocapitalize="none" value="{{ imported_localpart or '' }}" aria-describedby="{{ f.id }}-help" data-choose-username />
+        {{ field.error(error={"kind": "policy", "code": "username-invalid-chars"}, hidden=true) }}
 
         {% if f.errors is empty %}
           <div class="cpd-form-message cpd-form-help-message" id="{{ f.id }}-help">