Adds create option in filtered dropdown

In the filtered search dropdopwn adds a button
to enable creation of missing element.

app/assets/javascripts/vue_shared/components/filtered_search_dropdown.vue

 <script>
 import $ from 'jquery';
+import { GlButton } from '@gitlab/ui';
+import { __ } from '~/locale';
 import Icon from '~/vue_shared/components/icon.vue';
 /**
  * Renders a split dropdown with
  * an input that allows to search through the given
  * array of options.
+ *
+ * When there are no results and `showCreateMode` is true
+ * it renders a create button with the value typed.
  */
 export default {
   name: 'FilteredSearchDropdown',
   components: {
     Icon,
+    GlButton,
   },
   props: {
     title: {
@@ -43,6 +49,16 @@ export default {
       type: String,
       required: true,
     },
+    showCreateMode: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    createButtonText: {
+      type: String,
+      required: false,
+      default: __('Create'),
+    },
   },
   data() {
     return {
@@ -64,6 +80,12 @@ export default {
 
       return this.items.slice(0, this.visibleItems);
     },
+    computedCreateButtonText() {
+      return `${this.createButtonText} ${this.filter}`;
+    },
+    shouldRenderCreateButton() {
+      return this.showCreateMode && this.filteredResults.length === 0 && this.filter !== '';
+    },
   },
   mounted() {
     /**
@@ -112,10 +134,20 @@ export default {
         <div class="dropdown-content">
           <ul>
             <li v-for="(result, i) in filteredResults" :key="i" class="js-filtered-dropdown-result">
-              <slot name="result" :result="result"> {{ result[filterKey] }} </slot>
+              <slot name="result" :result="result">{{ result[filterKey] }}</slot>
             </li>
           </ul>
         </div>
+
+        <div v-if="shouldRenderCreateButton" class="dropdown-footer">
+          <slot name="footer" :filter="filter">
+            <gl-button
+              class="js-dropdown-create-button btn-transparent"
+              @click="$emit('createItem', filter)"
+              >{{ computedCreateButtonText }}</gl-button
+            >
+          </slot>
+        </div>
       </div>
     </div>
   </div>

spec/javascripts/vue_shared/components/filtered_search_dropdown_spec.js

@@ -88,4 +88,103 @@ describe('Filtered search dropdown', () => {
       });
     });
   });
+
+  describe('with create mode enabled', () => {
+    describe('when there are no matches', () => {
+      beforeEach(() => {
+        vm = mountComponent(Component, {
+          items: [
+            { title: 'One' },
+            { title: 'Two/three' },
+            { title: 'Three four' },
+            { title: 'Five' },
+          ],
+          filterKey: 'title',
+          showCreateMode: true,
+        });
+
+        vm.$el.querySelector('.js-filtered-dropdown-input').value = 'eleven';
+        vm.$el.querySelector('.js-filtered-dropdown-input').dispatchEvent(new Event('input'));
+      });
+
+      it('renders a create button', done => {
+        vm.$nextTick(() => {
+          expect(vm.$el.querySelector('.js-dropdown-create-button')).not.toBeNull();
+          done();
+        });
+      });
+
+      it('renders computed button text', done => {
+        vm.$nextTick(() => {
+          expect(vm.$el.querySelector('.js-dropdown-create-button').textContent.trim()).toEqual(
+            'Create eleven',
+          );
+          done();
+        });
+      });
+
+      describe('on click create button', () => {
+        it('emits createItem event with the filter', done => {
+          spyOn(vm, '$emit');
+          vm.$nextTick(() => {
+            vm.$el.querySelector('.js-dropdown-create-button').click();
+
+            expect(vm.$emit).toHaveBeenCalledWith('createItem', 'eleven');
+            done();
+          });
+        });
+      });
+    });
+
+    describe('when there are matches', () => {
+      beforeEach(() => {
+        vm = mountComponent(Component, {
+          items: [
+            { title: 'One' },
+            { title: 'Two/three' },
+            { title: 'Three four' },
+            { title: 'Five' },
+          ],
+          filterKey: 'title',
+          showCreateMode: true,
+        });
+
+        vm.$el.querySelector('.js-filtered-dropdown-input').value = 'one';
+        vm.$el.querySelector('.js-filtered-dropdown-input').dispatchEvent(new Event('input'));
+      });
+
+      it('does not render a create button', done => {
+        vm.$nextTick(() => {
+          expect(vm.$el.querySelector('.js-dropdown-create-button')).toBeNull();
+          done();
+        });
+      });
+    });
+  });
+
+  describe('with create mode disabled', () => {
+    describe('when there are no matches', () => {
+      beforeEach(() => {
+        vm = mountComponent(Component, {
+          items: [
+            { title: 'One' },
+            { title: 'Two/three' },
+            { title: 'Three four' },
+            { title: 'Five' },
+          ],
+          filterKey: 'title',
+        });
+
+        vm.$el.querySelector('.js-filtered-dropdown-input').value = 'eleven';
+        vm.$el.querySelector('.js-filtered-dropdown-input').dispatchEvent(new Event('input'));
+      });
+
+      it('does not render a create button', done => {
+        vm.$nextTick(() => {
+          expect(vm.$el.querySelector('.js-dropdown-create-button')).toBeNull();
+          done();
+        });
+      });
+    });
+  });
 });