Move karma spec to jest

Spec was failing on karma

app/assets/javascripts/diffs/components/compare_versions_dropdown.vue

 <script>
 import Icon from '~/vue_shared/components/icon.vue';
 import { n__, __, sprintf } from '~/locale';
+import { getParameterByName, parseBoolean } from '~/lib/utils/common_utils';
 import TimeAgo from '~/vue_shared/components/time_ago_tooltip.vue';
 
 export default {
@@ -94,6 +95,9 @@ export default {
       }
       return version.versionIndex === -1;
     },
+    isHead() {
+      return parseBoolean(getParameterByName('diff_head'));
+    },
     isLatest(version) {
       return (
         this.mergeRequestVersion && version.version_index === this.targetVersions[0].version_index
@@ -121,7 +125,8 @@ export default {
               <div>
                 <strong>
                   {{ versionName(version) }}
-                  <template v-if="isBase(version)">{{
+                  <template v-if="isHead()">{{ s__('DiffsCompareBaseBranch|(HEAD)') }}</template>
+                  <template v-else-if="isBase(version)">{{
                     s__('DiffsCompareBaseBranch|(base)')
                   }}</template>
                 </strong>

changelogs/unreleased/jdb-display-base-label-versions-dropdown.yml

+---
+title: Display base label in versions drop down
+merge_request: 25834
+author:
+type: added

locale/gitlab.pot

@@ -6700,6 +6700,9 @@ msgstr ""
 msgid "Difference between start date and now"
 msgstr ""
 
+msgid "DiffsCompareBaseBranch|(HEAD)"
+msgstr ""
+
 msgid "DiffsCompareBaseBranch|(base)"
 msgstr ""
 

spec/javascripts/diffs/components/compare_versions_dropdown_spec.js → spec/frontend/diffs/components/compare_versions_dropdown_spec.js

@@ -2,6 +2,7 @@ import { shallowMount, createLocalVue } from '@vue/test-utils';
 import CompareVersionsDropdown from '~/diffs/components/compare_versions_dropdown.vue';
 import diffsMockData from '../mock_data/merge_request_diffs';
 import TimeAgo from '~/vue_shared/components/time_ago_tooltip.vue';
+import { TEST_HOST } from 'helpers/test_constants';
 
 const localVue = createLocalVue();
 const targetBranch = { branchName: 'tmp-wine-dev', versionIndex: -1 };
@@ -109,6 +110,24 @@ describe('CompareVersionsDropdown', () => {
 
       expect(findLastLink().attributes('href')).toEqual(baseVersionPath);
       expect(findLastLink().text()).toContain('(base)');
+      expect(findLastLink().text()).not.toContain('(HEAD)');
+    });
+
+    it('should render a correct head version link', () => {
+      Object.defineProperty(window, 'location', {
+        writable: true,
+        value: { href: `${TEST_HOST}?diff_head=true` },
+      });
+
+      createComponent({
+        baseVersionPath,
+        otherVersions: diffsMockData.slice(1),
+        targetBranch,
+      });
+
+      expect(findLastLink().attributes('href')).toEqual(baseVersionPath);
+      expect(findLastLink().text()).not.toContain('(base)');
+      expect(findLastLink().text()).toContain('(HEAD)');
     });
 
     it('should not render commits count if no showCommitsCount is passed', () => {