Support backup GOV.UK Frontend if plugin is uninstalled

Adds an option param to `getNunjucksAppEnv()` to append backup view directories

lib/errorServer.js

@@ -102,17 +102,13 @@ function runErrorServer (error) {
     res.setHeader('Content-Type', 'text/html')
     res.writeHead(500)
 
-    // Get GOV.UK Frontend (internal) views
-    const govukFrontendNunjucksPaths = [govukFrontendInternal.config.nunjucksPaths].flat()
-      .map(nunjucksPath => path.join(govukFrontendInternal.baseDir, nunjucksPath))
-
     const fileContentsParts = []
 
     try {
-      const nunjucksAppEnv = getNunjucksAppEnv([
-        path.join(__dirname, 'nunjucks'),
-        ...govukFrontendNunjucksPaths
-      ])
+      const nunjucksAppEnv = getNunjucksAppEnv(
+        [path.join(__dirname, 'nunjucks')],
+        govukFrontendInternal // Add GOV.UK Frontend paths to Nunjucks views
+      )
       res.end(nunjucksAppEnv.render('views/error-handling/server-error', {
         govukFrontendInternal, // Add GOV.UK Frontend paths to Nunjucks context
         ...getErrorModel(error)

lib/nunjucks/nunjucksConfiguration.js

@@ -1,11 +1,32 @@
 const path = require('path')
-const nunjucks = require('nunjucks')
+const { Environment } = require('nunjucks')
 const NunjucksLoader = require('./nunjucksLoader')
 const { stopWatchingNunjucks } = NunjucksLoader
 const { startPerformanceTimer, endPerformanceTimer } = require('../utils/performance')
 
-function getNunjucksAppEnv (appViews) {
-  return new nunjucks.Environment(new NunjucksLoader(appViews))
+/**
+ * Create Nunjucks environment
+ *
+ * Provide an optional GOV.UK Frontend paths object to append
+ * backup view directories if the plugin version is uninstalled
+ *
+ * @param {string[]} appViews
+ * @param {GOVUKFrontendPaths} [govukFrontend]
+ * @returns {import('nunjucks').Environment}
+ */
+function getNunjucksAppEnv (appViews, govukFrontend) {
+  const nunjucksViews = [...appViews]
+
+  if (govukFrontend) {
+    const { baseDir, config } = govukFrontend
+
+    // Combine with backup GOV.UK Frontend views
+    nunjucksViews.push(...[config.nunjucksPaths].flat()
+      .map(nunjucksPath => path.join(baseDir, nunjucksPath))
+    )
+  }
+
+  return new Environment(new NunjucksLoader(nunjucksViews))
 }
 
 function expressNunjucks (env, app) {
@@ -33,3 +54,7 @@ function expressNunjucks (env, app) {
 }
 
 module.exports = { NunjucksLoader, getNunjucksAppEnv, expressNunjucks, stopWatchingNunjucks }
+
+/**
+ * @typedef {import('../govukFrontendPaths').GOVUKFrontendPaths} GOVUKFrontendPaths
+ */

server.js

@@ -92,8 +92,11 @@ if (config.isDevelopment) {
 
 nunjucksConfig.express = app
 
+// Finds GOV.UK Frontend via `getAppViews()` only if installed
+// but uses the internal package as a backup if uninstalled
 const nunjucksAppEnv = getNunjucksAppEnv(
-  plugins.getAppViews([appViewsDir, finalBackupNunjucksDir])
+  plugins.getAppViews([appViewsDir, finalBackupNunjucksDir]),
+  govukFrontendInternal
 )
 
 expressNunjucks(nunjucksAppEnv, app)