Flash is now a ES6 module

Reduced the technical debt around our JS flash function by making it a
module that is imported rather than relying on the global function.
The global function still exists mainly for technical debt with how
some requests are being completed, but new JS should import the module
directly.

Also reduces some tech debt in the file by removing the need for jQuery.
Instead Flash is now 100% vanilla JS.

app/assets/javascripts/flash.js

-/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, one-var, one-var-declaration-per-line, no-param-reassign, quotes, quote-props, prefer-template, comma-dangle, max-len */
-window.Flash = (function() {
-  var hideFlash;
-  hideFlash = function() {
-    return $(this).fadeOut();
-  };
-  /**
-   * Flash banner supports different types of Flash configurations
-   * along with ability to provide actionConfig which can be used to show
-   * additional action or link on banner next to message
-   *
-   * @param {String} message Flash message
-   * @param {String} type Type of Flash, it can be `notice` or `alert` (default)
-   * @param {Object} parent Reference to Parent element under which Flash needs to appear
-   * @param {Object} actionConfig Map of config to show action on banner
-   *    @param {String} href URL to which action link should point (default '#')
-   *    @param {String} title Title of action
-   *    @param {Function} clickHandler Method to call when action is clicked on
-   */
-  function Flash(message, type, parent, actionConfig) {
-    var flash, textDiv, actionLink;
-    if (type == null) {
-      type = 'alert';
-    }
-    if (parent == null) {
-      parent = null;
-    }
-    if (parent) {
-      this.flashContainer = parent.find('.flash-container');
-    } else {
-      this.flashContainer = $('.flash-container-page');
-    }
-    this.flashContainer.html('');
-    flash = $('<div/>', {
-      "class": "flash-" + type
-    });
-    flash.on('click', hideFlash);
-    textDiv = $('<div/>', {
-      "class": 'flash-text',
-      text: message
-    });
-    textDiv.appendTo(flash);
-    if (actionConfig) {
-      const actionLinkConfig = {
-        class: 'flash-action',
-        href: actionConfig.href || '#',
-        text: actionConfig.title
-      };
-      if (!actionConfig.href) {
-        actionLinkConfig.role = 'button';
-      }
-      actionLink = $('<a/>', actionLinkConfig);
-      actionLink.appendTo(flash);
-      this.flashContainer.on('click', '.flash-action', actionConfig.clickHandler);
-    }
-    if (this.flashContainer.parent().hasClass('content-wrapper')) {
-      textDiv.addClass('container-fluid container-limited');
+import _ from 'underscore';
+const hideFlash = (flashEl) => {
+  flashEl.style.transition = 'opacity .3s'; // eslint-disable-line no-param-reassign
+  flashEl.style.opacity = '0'; // eslint-disable-line no-param-reassign
+  flashEl.addEventListener('transitionend', () => {
+    flashEl.remove();
+  }, {
+    once: true,
+  });
+};
+const createAction = config => `
+  <a
+    href="${config.href || '#'}"
+    class="flash-action"
+    ${config.href ? 'role="button"' : ''}
+  >
+    ${_.escape(config.title)}
+  </a>
+`;
+const createFlashEl = (message, type) => `
+  <div
+    class="flash-${type}"
+  >
+    <div
+      class="flash-text"
+    >
+      ${_.escape(message)}
+    </div>
+  </div>
+`;
+const Flash = function Flash(message, type = 'alert', parent = document, actionConfig = null) {
+  const flashContainer = parent.querySelector('.flash-container');
+  flashContainer.innerHTML = createFlashEl(message, type);
+  const flashEl = flashContainer.querySelector(`.flash-${type}`);
+  flashEl.addEventListener('click', () => hideFlash(flashEl));
+  if (actionConfig) {
+    flashEl.innerHTML += createAction(actionConfig);
+
+    if (actionConfig.clickHandler) {
+      flashEl.querySelector('.flash-action').addEventListener('click', e => actionConfig.clickHandler(e));
     }
-    flash.appendTo(this.flashContainer);
-    this.flashContainer.show();
   }
-  return Flash;
-})();
+  if (flashContainer.parentNode.classList.contains('content-wrapper')) {
+    const flashText = flashEl.querySelector('.flash-text');
+
+    flashText.classList.add('container-fluid');
+    flashText.classList.add('container-limited');
+  }
+
+  flashContainer.style.display = 'block';
+
+  return flashContainer;
+};
+
+export {
+  Flash as default,
+  createFlashEl,
+  hideFlash,
+};
+window.Flash = Flash;

app/assets/javascripts/groups/index.js

-/* global Flash */
-
 import Vue from 'vue';
+import Flash from '../flash';
 import GroupFilterableList from './groups_filterable_list';
 import GroupsComponent from './components/groups.vue';
 import GroupFolder from './components/group_folder.vue';

app/assets/javascripts/integrations/integration_settings_form.js

-/* global Flash */
+import Flash from '../flash';
 export default class IntegrationSettingsForm {
   constructor(formSelector) {
@@ -102,7 +102,7 @@ export default class IntegrationSettingsForm {
     })
     .done((res) => {
       if (res.error) {
-        new Flash(`${res.message} ${res.service_response}`, null, null, {
+        new Flash(`${res.message} ${res.service_response}`, 'alert', document, {
           title: 'Save anyway',
           clickHandler: (e) => {
             e.preventDefault();

app/assets/javascripts/issuable_bulk_update_actions.js

 /* eslint-disable comma-dangle, quotes, consistent-return, func-names, array-callback-return, space-before-function-paren, prefer-arrow-callback, max-len, no-unused-expressions, no-sequences, no-underscore-dangle, no-unused-vars, no-param-reassign */
 /* global IssuableIndex */
-/* global Flash */
 import _ from 'underscore';
+import Flash from './flash';
 export default {
   init({ container, form, issues, prefixId } = {}) {

app/assets/javascripts/issue.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-rest-params, wrap-iife, one-var, no-underscore-dangle, one-var-declaration-per-line, object-shorthand, no-unused-vars, no-new, comma-dangle, consistent-return, quotes, dot-notation, quote-props, prefer-arrow-callback, max-len */
-/* global Flash */
-
 import 'vendor/jquery.waitforimages';
 import '~/lib/utils/text_utility';
-import './flash';
+import Flash from './flash';
 import TaskList from './task_list';
 import CreateMergeRequestDropdown from './create_merge_request_dropdown';
 import IssuablesHelper from './helpers/issuables_helper';

app/assets/javascripts/issue_show/components/app.vue

 <script>
-/* global Flash */
 import Visibility from 'visibilityjs';
+import Flash from '../../flash';
 import Poll from '../../lib/utils/poll';
 import eventHub from '../event_hub';
 import Service from '../services/index';

app/assets/javascripts/jobs/job_details_mediator.js

-/* global Flash */
 /* global Build */
 import Visibility from 'visibilityjs';
+import Flash from '../flash';
 import Poll from '../lib/utils/poll';
 import JobStore from './stores/job_store';
 import JobService from './services/job_service';

app/assets/javascripts/label_manager.js

 /* eslint-disable comma-dangle, class-methods-use-this, no-underscore-dangle, no-param-reassign, no-unused-vars, consistent-return, func-names, space-before-function-paren, max-len */
-/* global Flash */
 /* global Sortable */
+import Flash from './flash';
+
 ((global) => {
   class LabelManager {
     constructor({ togglePriorityButton, prioritizedLabels, otherLabels } = {}) {

app/assets/javascripts/main.js

 /* eslint-disable func-names, space-before-function-paren, no-var, quotes, consistent-return, prefer-arrow-callback, comma-dangle, object-shorthand, no-new, max-len, no-multi-spaces, import/newline-after-import, import/first */
-/* global Flash */
 /* global ConfirmDangerModal */
 /* global Aside */
@@ -75,7 +74,7 @@ import './diff';
 import './dropzone_input';
 import './due_date_select';
 import './files_comment_button';
-import './flash';
+import Flash from './flash';
 import './gl_dropdown';
 import './gl_field_error';
 import './gl_field_errors';
@@ -168,7 +167,6 @@ $(function () {
   var $document = $(document);
   var $window = $(window);
   var $sidebarGutterToggle = $('.js-sidebar-toggle');
-  var $flash = $('.flash-container');
   var bootstrapBreakpoint = bp.getBreakpointSize();
   var fitSidebarForSize;
@@ -253,13 +251,6 @@ $(function () {
   // Form submitter
   });
   gl.utils.localTimeAgo($('abbr.timeago, .js-timeago'), true);
-  // Flash
-  if ($flash.length > 0) {
-    $flash.click(function () {
-      return $(this).fadeOut();
-    });
-    $flash.show();
-  }
   // Disable form buttons while a form is submitting
   $body.on('ajax:complete, ajax:beforeSend, submit', 'form', function (e) {
     var buttons;

app/assets/javascripts/merge_conflicts/components/diff_file_editor.js

 /* eslint-disable comma-dangle, quote-props, no-useless-computed-key, object-shorthand, no-new, no-param-reassign, max-len */
 /* global ace */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../../flash';
 ((global) => {
   global.mergeConflicts = global.mergeConflicts || {};

app/assets/javascripts/merge_conflicts/merge_conflicts_bundle.js

 /* eslint-disable new-cap, comma-dangle, no-new */
-/* global Flash */
 import Vue from 'vue';
+import Flash from '../flash';
 import initIssuableSidebar from '../init_issuable_sidebar';
 import './merge_conflict_store';
 import './merge_conflict_service';

app/assets/javascripts/merge_request_tabs.js

 /* eslint-disable no-new, class-methods-use-this */
-/* global Flash */
 /* global notes */
 import Cookies from 'js-cookie';
-import './flash';
+import Flash from './flash';
 import BlobForkSuggestion from './blob/blob_fork_suggestion';
 import initChangesDropdown from './init_changes_dropdown';
 import bp from './breakpoints';

app/assets/javascripts/milestone.js

 /* eslint-disable func-names, space-before-function-paren, wrap-iife, no-use-before-define, camelcase, quotes, object-shorthand, no-shadow, no-unused-vars, comma-dangle, no-var, prefer-template, no-underscore-dangle, consistent-return, one-var, one-var-declaration-per-line, default-case, prefer-arrow-callback, max-len */
-/* global Flash */
 /* global Sortable */
+import Flash from './flash';
+
 (function() {
   this.Milestone = (function() {
     function Milestone() {

app/assets/javascripts/mini_pipeline_graph_dropdown.js

 /* eslint-disable no-new */
-/* global Flash */
+import Flash from './flash';
 /**
  * In each pipelines table we have a mini pipeline graph for each pipeline.

app/assets/javascripts/monitoring/components/dashboard.vue

 <script>
-  /* global Flash */
   import _ from 'underscore';
+  import Flash from '../../flash';
   import MonitoringService from '../services/monitoring_service';
   import GraphGroup from './graph_group.vue';
   import Graph from './graph.vue';

app/assets/javascripts/notes.js

@@ -5,7 +5,6 @@ default-case, prefer-template, consistent-return, no-alert, no-return-assign,
 no-param-reassign, prefer-arrow-callback, no-else-return, comma-dangle, no-new,
 brace-style, no-lonely-if, vars-on-top, no-unused-vars, no-sequences, no-shadow,
 newline-per-chained-call, no-useless-escape, class-methods-use-this */
-/* global Flash */
 /* global Autosave */
 /* global ResolveService */
 /* global mrRefreshWidgetUrl */
@@ -18,6 +17,7 @@ import Dropzone from 'dropzone';
 import 'vendor/jquery.caret'; // required by jquery.atwho
 import 'vendor/jquery.atwho';
 import AjaxCache from '~/lib/utils/ajax_cache';
+import Flash from './flash';
 import CommentTypeToggle from './comment_type_toggle';
 import loadAwardsHandler from './awards_handler';
 import './autosave';
@@ -354,7 +354,7 @@ export default class Notes {
             Object.keys(noteEntity.commands_changes).length > 0) {
           $notesList.find('.system-note.being-posted').remove();
         }
-        this.addFlash(noteEntity.errors.commands_only, 'notice', this.parentTimeline);
+        this.addFlash(noteEntity.errors.commands_only, 'notice', this.parentTimeline); // TODO: CHECK THIS!
         this.refresh();
       }
       return;

app/assets/javascripts/notes/components/issue_comment_form.vue

 <script>
-  /* global Flash, Autosave */
+  /* global Autosave */
   import { mapActions, mapGetters } from 'vuex';
   import _ from 'underscore';
   import autosize from 'vendor/autosize';
+  import Flash from '../../flash';
   import '../../autosave';
   import TaskList from '../../task_list';
   import * as constants from '../constants';
@@ -145,7 +146,7 @@
                   Flash(
                     'Something went wrong while adding your comment. Please try again.',
                     'alert',
-                    $(this.$refs.commentForm),
+                    this.$refs.commentForm,
                   );
                 }
               } else {
@@ -160,7 +161,7 @@
               this.isSubmitting = false;
               this.discard(false);
               const msg = 'Your comment could not be submitted! Please check your network connection and try again.';
-              Flash(msg, 'alert', $(this.$el));
+              Flash(msg, 'alert', this.$el);
               this.note = noteData.data.note.note; // Restore textarea content.
               this.removePlaceholderNotes();
             });

app/assets/javascripts/notes/components/issue_discussion.vue

 <script>
-  /* global Flash */
   import { mapActions, mapGetters } from 'vuex';
+  import Flash from '../../flash';
   import { SYSTEM_NOTE } from '../constants';
   import issueNote from './issue_note.vue';
   import userAvatarLink from '../../vue_shared/components/user_avatar/user_avatar_link.vue';
@@ -133,7 +133,7 @@
             this.isReplying = true;
             this.$nextTick(() => {
               const msg = 'Your comment could not be submitted! Please check your network connection and try again.';
-              Flash(msg, 'alert', $(this.$el));
+              Flash(msg, 'alert', this.$el);
               this.$refs.noteForm.note = noteText;
               callback(err);
             });

app/assets/javascripts/notes/components/issue_note.vue

 <script>
-  /* global Flash */
-
   import { mapGetters, mapActions } from 'vuex';
+  import Flash from '../../flash';
   import userAvatarLink from '../../vue_shared/components/user_avatar/user_avatar_link.vue';
   import issueNoteHeader from './issue_note_header.vue';
   import issueNoteActions from './issue_note_actions.vue';
@@ -101,7 +100,7 @@
             this.isEditing = true;
             this.$nextTick(() => {
               const msg = 'Something went wrong while editing your comment. Please try again.';
-              Flash(msg, 'alert', $(this.$el));
+              Flash(msg, 'alert', this.$el);
               this.recoverNoteContent(noteText);
               callback();
             });

app/assets/javascripts/notes/components/issue_note_awards_list.vue

 <script>
-  /* global Flash */
-
   import { mapActions, mapGetters } from 'vuex';
   import emojiSmiling from 'icons/_emoji_slightly_smiling_face.svg';
   import emojiSmile from 'icons/_emoji_smile.svg';
   import emojiSmiley from 'icons/_emoji_smiley.svg';
+  import Flash from '../../flash';
   import { glEmojiTag } from '../../emoji';
   import tooltip from '../../vue_shared/directives/tooltip';