Fix lint errors, add constructor info

app/assets/javascripts/protected_branches/protected_branch_dropdown.js

-/* eslint-disable comma-dangle, no-unused-vars */
-
-class ProtectedBranchDropdown {
+export default class ProtectedBranchDropdown {
+  /**
+   * @param {Object} options containing
+   *                         `$dropdown` target element
+   *                          `onSelect` event callback
+   * $dropdown must be an element created using `dropdown_branch()` rails helper
+   */
   constructor(options) {
     this.onSelect = options.onSelect;
     this.$dropdown = options.$dropdown;
@@ -12,7 +16,7 @@ class ProtectedBranchDropdown {
     this.bindEvents();
 
     // Hide footer
-    this.$dropdownFooter.addClass('hidden');
+    this.toggleFooter(true);
   }
 
   buildDropdown() {
@@ -21,7 +25,7 @@ class ProtectedBranchDropdown {
       filterable: true,
       remote: false,
       search: {
-        fields: ['title']
+        fields: ['title'],
       },
       selectable: true,
       toggleLabel(selected) {
@@ -36,10 +40,9 @@ class ProtectedBranchDropdown {
       },
       onFilter: this.toggleCreateNewButton.bind(this),
       clicked: (options) => {
-        const { $el, e } = options;
-        e.preventDefault();
+        options.e.preventDefault();
         this.onSelect();
-      }
+      },
     });
   }
 
@@ -64,20 +67,22 @@ class ProtectedBranchDropdown {
   }
 
   toggleCreateNewButton(branchName) {
-    this.selectedBranch = {
-      title: branchName,
-      id: branchName,
-      text: branchName
-    };
-
     if (branchName) {
+      this.selectedBranch = {
+        title: branchName,
+        id: branchName,
+        text: branchName,
+      };
+
       this.$dropdownContainer
         .find('.js-create-new-protected-branch code')
         .text(branchName);
     }
 
-    this.$dropdownFooter.toggleClass('hidden', !branchName);
+    this.toggleFooter(!branchName);
   }
-}
 
-window.ProtectedBranchDropdown = ProtectedBranchDropdown;
+  toggleFooter(toggleState) {
+    this.$dropdownFooter.toggleClass('hidden', toggleState);
+  }
+}