Improve require input spec

spec/javascripts/behaviors/requires_input_spec.js

-/* eslint-disable space-before-function-paren, no-var */
-
 import '~/behaviors/requires_input';
 
-(function() {
-  describe('requiresInput', function() {
+describe('requiresInput', () => {
+  let submitButton;
   preloadFixtures('branches/new_branch.html.raw');
-    beforeEach(function() {
+
+  beforeEach(() => {
     loadFixtures('branches/new_branch.html.raw');
-      this.submitButton = $('button[type="submit"]');
+    submitButton = $('button[type="submit"]');
   });
-    it('disables submit when any field is required', function() {
+
+  it('disables submit when any field is required', () => {
     $('.js-requires-input').requiresInput();
-      return expect(this.submitButton).toBeDisabled();
+    expect(submitButton).toBeDisabled();
   });
-    it('enables submit when no field is required', function() {
+
+  it('enables submit when no field is required', () => {
     $('*[required=required]').removeAttr('required');
     $('.js-requires-input').requiresInput();
-      return expect(this.submitButton).not.toBeDisabled();
+    expect(submitButton).not.toBeDisabled();
   });
-    it('enables submit when all required fields are pre-filled', function() {
+
+  it('enables submit when all required fields are pre-filled', () => {
     $('*[required=required]').remove();
     $('.js-requires-input').requiresInput();
-      return expect($('.submit')).not.toBeDisabled();
+    expect($('.submit')).not.toBeDisabled();
   });
-    it('enables submit when all required fields receive input', function() {
+
+  it('enables submit when all required fields receive input', () => {
     $('.js-requires-input').requiresInput();
     $('#required1').val('input1').change();
-      expect(this.submitButton).toBeDisabled();
+    expect(submitButton).toBeDisabled();
+
     $('#optional1').val('input1').change();
-      expect(this.submitButton).toBeDisabled();
+    expect(submitButton).toBeDisabled();
+
     $('#required2').val('input2').change();
     $('#required3').val('input3').change();
     $('#required4').val('input4').change();
     $('#required5').val('1').change();
-      return expect($('.submit')).not.toBeDisabled();
-    });
+    expect($('.submit')).not.toBeDisabled();
   });
-}).call(window);
+});