6 files declare their use of AlertCommand

AjaxCommandsTest.php in core/tests/Drupal/Tests/Core/Ajax/AjaxCommandsTest.php
AjaxRenderer.php in core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
AjaxTestController.php in core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
ajax_forms_test.module in core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
Mock module for Ajax forms testing.
FormAjaxResponseBuilderTest.php in core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php
FrameworkTest.php in core/modules/system/tests/src/Functional/Ajax/FrameworkTest.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.