Move cleanForSlug into its own utils module

This allows us to test it without having to jump through hoops to make the rest of page-editor.js module-friendly, and moving the test module out of client/src/entrypoints means we don't duplicate it into wagtail/admin/static (where the test runner tries to run it again with broken imports)

client/src/entrypoints/admin/page-editor.js

@@ -1,4 +1,5 @@
 import $ from 'jquery';
+import { cleanForSlug } from '../../utils/cleanForSlug';
 
 window.halloPlugins = {};
 
@@ -184,26 +185,6 @@ function InlinePanel(opts) {  // lgtm[js/unused-local-variable]
 }
 window.InlinePanel = InlinePanel;
 
-function cleanForSlug(val, useURLify) {
-  if (useURLify) {
-    // URLify performs extra processing on the string (e.g. removing stopwords) and is more suitable
-    // for creating a slug from the title, rather than sanitising a slug entered manually
-    // eslint-disable-next-line no-undef, new-cap
-    const cleaned = URLify(val, 255, window.unicodeSlugsEnabled);
-
-    // if the result is blank (e.g. because the title consisted entirely of stopwords),
-    // fall through to the non-URLify method
-    if (cleaned) {
-      return cleaned;
-    }
-  }
-
-  // just do the "replace"
-  if (window.unicodeSlugsEnabled) {
-    return val.replace(/\s/g, '-').replace(/[&/\\#,+()$~%.'":`@^!*?<>{}]/g, '').toLowerCase();
-  }
-  return val.replace(/\s/g, '-').replace(/[^A-Za-z0-9\-_]/g, '').toLowerCase();
-}
 window.cleanForSlug = cleanForSlug;
 
 function initSlugAutoPopulate() {
@@ -425,7 +406,3 @@ window.updateFooterSaveWarning = (formDirty, commentsDirty) => {
     updateWarnings();
   }
 };
-
-if (typeof module !== 'undefined' && module.exports) {
-  module.exports.cleanForSlug = cleanForSlug;
-}

client/src/utils/cleanForSlug.js

@@ -0,0 +1,20 @@
+export function cleanForSlug(val, useURLify) {
+  if (useURLify) {
+    // URLify performs extra processing on the string (e.g. removing stopwords) and is more suitable
+    // for creating a slug from the title, rather than sanitising a slug entered manually
+    // eslint-disable-next-line no-undef, new-cap
+    const cleaned = URLify(val, 255, window.unicodeSlugsEnabled);
+
+    // if the result is blank (e.g. because the title consisted entirely of stopwords),
+    // fall through to the non-URLify method
+    if (cleaned) {
+      return cleaned;
+    }
+  }
+
+  // just do the "replace"
+  if (window.unicodeSlugsEnabled) {
+    return val.replace(/\s/g, '-').replace(/[&/\\#,+()$~%.'":`@^!*?<>{}]/g, '').toLowerCase();
+  }
+  return val.replace(/\s/g, '-').replace(/[^A-Za-z0-9\-_]/g, '').toLowerCase();
+}

client/src/utils/cleanForSlug.test.js

@@ -1,12 +1,6 @@
-window.$ = require('../../../../wagtail/admin/static_src/wagtailadmin/js/vendor/jquery-3.5.1.min');
 // eslint-disable-next-line no-unused-expressions
-require('../../../../wagtail/admin/static_src/wagtailadmin/js/vendor/urlify').default;
-
-window.comments = {
-  getContentPath: jest.fn(),
-};
-
-const cleanForSlug = require('./page-editor').cleanForSlug;
+require('../../../wagtail/admin/static_src/wagtailadmin/js/vendor/urlify').default;
+import { cleanForSlug } from './cleanForSlug';
 
 describe('page-editor tests', () => {
   describe('cleanForSlug without unicode slugs enabled', () => {