Add enforcement of .js extensions for require() (#1586)

DEFRA/water-abstraction-team#115

Recently, we [overhauled our project linting](#1476). We moved to ESLint v9 and its new flat file config, switched to Prettier to handle our 'style' conventions, and replaced StandardJS with Neostandard.

On that last point, the one rule we could not migrate at the time was the enforcement of `.js` extensions when requiring modules. We know Node.js will handle this for us and that the extension is not required. Eventually, we intend to migrate to ESM modules, and from previous attempts, having the `.js` in place reduces the amount of work this requires.

We are now happy to report that in v0.12.0 [Neostandard](https://github.com/neostandard/neostandard) has adopted [eslint-plugin-import-x](https://github.com/un-ts/eslint-plugin-import-x) so we can add the rule back in, which is handy, as we've spotted new instances of folks forgetting to do this!

.labrc.js

@@ -59,6 +59,9 @@ module.exports = {
     // whilst avoiding having to pass it around
     'GlobalNotifier',
     // HapiServerMethods is added by us in a server plugin to allow us to access server methods globally.
-    'HapiServerMethods'
+    'HapiServerMethods',
+    // Caused by eslint-plugin-import-x which is a dependence of Neostandard we rely on to ensure the team adds the .js
+    // extension to files when requiring them
+    '__rewriteRelativeImportExtension'
   ].join(',')
 }

app/controllers/notifications-setup.controller.js

@@ -2,7 +2,7 @@
 
 const ReturnsPeriodService = require('../services/notifications/setup/returns-period.service.js')
 const SubmitReturnsPeriodService = require('../services/notifications/setup/submit-returns-period.service.js')
-const InitiateSessionService = require('../services/notifications/setup/initiate-session.service')
+const InitiateSessionService = require('../services/notifications/setup/initiate-session.service.js')
 
 /**
  * Controller for /notifications/setup endpoints

app/presenters/notifications/setup/returns-period.presenter.js

@@ -5,8 +5,8 @@
  * @module ReturnsPeriodPresenter
  */
 
-const { determineUpcomingReturnPeriods } = require('../../../lib/return-periods.lib')
-const { formatLongDate } = require('../../base.presenter')
+const { determineUpcomingReturnPeriods } = require('../../../lib/return-periods.lib.js')
+const { formatLongDate } = require('../../base.presenter.js')
 
 /**
  * Formats data for the `/notifications/setup/returns-period` page

app/services/jobs/import/process-import-licence.service.js

@@ -5,9 +5,9 @@
  * @module ProcessImportLicence
  */
 
-const DetermineLicenceEndDateChangedService = require('./determine-licence-end-date-changed.service')
-const ProcessBillingFlagService = require('../../licences/supplementary/process-billing-flag.service')
-const GenerateReturnLogsService = require('./generate-return-logs.service')
+const DetermineLicenceEndDateChangedService = require('./determine-licence-end-date-changed.service.js')
+const ProcessBillingFlagService = require('../../licences/supplementary/process-billing-flag.service.js')
+const GenerateReturnLogsService = require('./generate-return-logs.service.js')
 
 /**
  * Process licence for the licences imported from NALD

app/services/notifications/setup/returns-period.service.js

@@ -6,7 +6,7 @@
  */
 
 const NotificationsPresenter = require('../../../presenters/notifications/setup/returns-period.presenter.js')
-const SessionModel = require('../../../models/session.model')
+const SessionModel = require('../../../models/session.model.js')
 
 /**
  * Formats data for the `/notifications/setup/returns-period` page

app/services/notifications/setup/submit-returns-period.service.js

@@ -7,7 +7,7 @@
 
 const NotificationsPresenter = require('../../../presenters/notifications/setup/returns-period.presenter.js')
 const ReturnsPeriodValidator = require('../../../validators/notifications/setup/returns-periods.validator.js')
-const SessionModel = require('../../../models/session.model')
+const SessionModel = require('../../../models/session.model.js')
 
 /**
  * Formats data for the `/notifications/setup/returns-period` page

app/validators/notifications/setup/returns-periods.validator.js

@@ -6,7 +6,7 @@
  */
 
 const Joi = require('joi')
-const { returnPeriodDates } = require('../../../lib/static-lookups.lib')
+const { returnPeriodDates } = require('../../../lib/static-lookups.lib.js')
 
 const errorMessage = 'Select the returns periods for the invitations'
 

eslint.config.js

@@ -2,8 +2,8 @@
 
 const jsdocPlugin = require('eslint-plugin-jsdoc')
 const eslintPluginPrettierRecommended = require('eslint-plugin-prettier/recommended')
-const neostandard = require('neostandard')
 const globals = require('globals')
+const neostandard = require('neostandard')
 
 module.exports = [
   // Start with neostandard ESLint rules. neostandard is the successor to StandardJS (which has stalled due to a
@@ -36,11 +36,14 @@ module.exports = [
     ignores: ['docs/**/*'],
     plugins: {
       // https://github.com/gajus/eslint-plugin-jsdoc
-      jsdoc: jsdocPlugin
+      jsdoc: jsdocPlugin,
+      import: neostandard.plugins['import-x']
     },
     rules: {
       // Enforce braces around the function body of arrow functions
       'arrow-body-style': ['error', 'always'],
+      // Enforce .js extension when requiring files
+      'import/extensions': ['error', 'always'],
       // Enforce 'use strict' declarations in all modules
       strict: ['error', 'global'],
       'jsdoc/check-alignment': 'error',
@@ -71,6 +74,15 @@ module.exports = [
       'jsdoc/require-jsdoc': 'off'
     }
   },
+  // This section adds another override to the configuration object above. It tells the import/extensions plugin to
+  // ignore this file. The plugin flags `eslint-plugin-prettier/recommended` as a violation, even though it is not and
+  // adding `.js` causes it to break
+  {
+    files: ['eslint.config.js'],
+    rules: {
+      'import/extensions': 'off'
+    }
+  },
   // Adds prettier ESLint rules. It automatically sets up eslint-config-prettier, which turns off any rules declared
   // above that conflict with prettier. That shouldn't be any, as we tell neostandard not to include any style rules
   // and the ones we've declared we've done as per eslint-config-prettier docs on special rules. As recommended by