Switch to using gulp-better-rollup

Means our rollup bundling doesn't leave any
artefact files lying around that we'd then have to
deal with.

Also includes:
- removal of some JSHint config' marking the
  artefacts as scripts to ignore
- use of streamqueue package to allow the same
  ordering of scripts as before

gulpfile.js

@@ -6,16 +6,16 @@
 // 1. LIBRARIES
 // - - - - - - - - - - - - - - -
 const { src, pipe, dest, series, parallel, watch } = require('gulp');
-const rollup = require('rollup');
 const rollupPluginCommonjs = require('rollup-plugin-commonjs');
 const rollupPluginNodeResolve = require('rollup-plugin-node-resolve');
+const streamqueue = require('streamqueue');
 const stylish = require('jshint-stylish');
-const del = require('del');
 
 const plugins = {};
 plugins.addSrc = require('gulp-add-src');
 plugins.babel = require('gulp-babel');
 plugins.base64 = require('gulp-base64-inline');
+plugins.rollup = require('gulp-better-rollup')
 plugins.concat = require('gulp-concat');
 plugins.cssUrlAdjuster = require('gulp-css-url-adjuster');
 plugins.jshint = require('gulp-jshint');
@@ -54,70 +54,78 @@ const copy = {
 };
 
 
-const bundleJavaScriptModules = async function () {
-  const bundle = await rollup.rollup({
-    input: paths.src + 'javascripts/modules/all.mjs',
-    plugins: [
-      // determine module entry points from either 'module' or 'main' fields in package.json
-      rollupPluginNodeResolve({
-        mainFields: ['module', 'main']
-      }),
-      // gulp rollup runs on nodeJS so reads modules in commonJS format
-      // this adds node_modules to the require path so it can find the GOVUK Frontend modules
-      rollupPluginCommonjs({
-        include: 'node_modules/**'
-      })
-    ]
-  });
-
-  // write resulting module to Immediately Invoked Function Expression (IIFE) format
-  // map the exported code to the window.GOVUK namespace
-  await bundle.write({
-    file: paths.src + 'javascripts/modules/all.js',
-    format: 'iife',
-    name: 'GOVUK'
-  });
-};
 
 
 const javascripts = () => {
-  return src([
-      paths.toolkit + 'javascripts/govuk/modules.js',
-      paths.toolkit + 'javascripts/govuk/show-hide-content.js',
-      paths.src + 'javascripts/govuk/cookie-functions.js',
-      paths.src + 'javascripts/cookieMessage.js',
-      paths.src + 'javascripts/stick-to-window-when-scrolling.js',
-      paths.src + 'javascripts/detailsPolyfill.js',
-      paths.src + 'javascripts/apiKey.js',
-      paths.src + 'javascripts/autofocus.js',
-      paths.src + 'javascripts/enhancedTextbox.js',
-      paths.src + 'javascripts/fileUpload.js',
-      paths.src + 'javascripts/radioSelect.js',
-      paths.src + 'javascripts/updateContent.js',
-      paths.src + 'javascripts/listEntry.js',
-      paths.src + 'javascripts/liveSearch.js',
-      paths.src + 'javascripts/errorTracking.js',
-      paths.src + 'javascripts/preventDuplicateFormSubmissions.js',
-      paths.src + 'javascripts/fullscreenTable.js',
-      paths.src + 'javascripts/previewPane.js',
-      paths.src + 'javascripts/colourPreview.js',
-      paths.src + 'javascripts/templateFolderForm.js',
-      paths.src + 'javascripts/collapsibleCheckboxes.js',
-      paths.src + 'javascripts/main.js'
-    ])
-    .pipe(plugins.prettyerror())
-    .pipe(plugins.babel({
-      presets: ['@babel/preset-env']
-    }))
+  // JS from third-party sources
+  // We assume none of it will need to pass through Babel
+  const vendored = src(paths.src + 'javascripts/modules/all.mjs')
+    // Use Rollup to combine all JS in JS module format into a Immediately Invoked Function
+    // Expression (IIFE) to:
+    // - deliver it in one bundle
+    // - allow it to run in browsers without support for JS Modules
+    .pipe(plugins.rollup(
+      {
+        plugins: [
+          // determine module entry points from either 'module' or 'main' fields in package.json
+          rollupPluginNodeResolve({
+            mainFields: ['module', 'main']
+          }),
+          // gulp rollup runs on nodeJS so reads modules in commonJS format
+          // this adds node_modules to the require path so it can find the GOVUK Frontend modules
+          rollupPluginCommonjs({
+            include: 'node_modules/**'
+          })
+        ]
+      },
+      {
+        format: 'iife',
+        name: 'GOVUK'
+      }
+    ))
+    // return a stream which pipes these files before the JS modules bundle
     .pipe(plugins.addSrc.prepend([
       paths.npm + 'hogan.js/dist/hogan-3.0.2.js',
       paths.npm + 'jquery/dist/jquery.min.js',
       paths.npm + 'query-command-supported/dist/queryCommandSupported.min.js',
       paths.npm + 'diff-dom/diffDOM.js',
       paths.npm + 'timeago/jquery.timeago.js',
-      paths.npm + 'textarea-caret/index.js',
-      paths.src + 'javascripts/modules/all.js'
-    ]))
+      paths.npm + 'textarea-caret/index.js'
+    ]));
+
+  // JS local to this application
+  const local = src([
+    paths.toolkit + 'javascripts/govuk/modules.js',
+    paths.toolkit + 'javascripts/govuk/show-hide-content.js',
+    paths.src + 'javascripts/govuk/cookie-functions.js',
+    paths.src + 'javascripts/cookieMessage.js',
+    paths.src + 'javascripts/stick-to-window-when-scrolling.js',
+    paths.src + 'javascripts/detailsPolyfill.js',
+    paths.src + 'javascripts/apiKey.js',
+    paths.src + 'javascripts/autofocus.js',
+    paths.src + 'javascripts/enhancedTextbox.js',
+    paths.src + 'javascripts/fileUpload.js',
+    paths.src + 'javascripts/radioSelect.js',
+    paths.src + 'javascripts/updateContent.js',
+    paths.src + 'javascripts/listEntry.js',
+    paths.src + 'javascripts/liveSearch.js',
+    paths.src + 'javascripts/errorTracking.js',
+    paths.src + 'javascripts/preventDuplicateFormSubmissions.js',
+    paths.src + 'javascripts/fullscreenTable.js',
+    paths.src + 'javascripts/previewPane.js',
+    paths.src + 'javascripts/colourPreview.js',
+    paths.src + 'javascripts/templateFolderForm.js',
+    paths.src + 'javascripts/collapsibleCheckboxes.js',
+    paths.src + 'javascripts/main.js'
+  ])
+  .pipe(plugins.prettyerror())
+  .pipe(plugins.babel({
+    presets: ['@babel/preset-env']
+  }));
+
+  // return single stream of all vinyl objects piped from the end of the vendored stream, then
+  // those from the end of the local stream
+  return streamqueue({ objectMode: true }, vendored, local)
     .pipe(plugins.uglify())
     .pipe(plugins.concat('all.js'))
     .pipe(dest(paths.dist + 'javascripts/'))
@@ -188,8 +196,7 @@ const lint = {
   },
   'js': (cb) => {
     return src(
-        paths.src + 'javascripts/**/*.js',
-        { ignore: paths.src + 'javascripts/modules/*.js' } // ignore bundler boilerplate JS
+        paths.src + 'javascripts/**/*.js'
       )
       .pipe(plugins.jshint())
       .pipe(plugins.jshint.reporter(stylish))
@@ -198,13 +205,6 @@ const lint = {
 };
 
 
-const clean = {
-  javascripts: (cb) => {
-    return del([paths.src + 'javascripts/modules/all.js'])
-  }
-}
-
-
 // Default: compile everything
 const defaultTask = parallel(
   series(
@@ -214,9 +214,7 @@ const defaultTask = parallel(
   series(
     copy.error_pages,
     series(
-      bundleJavaScriptModules,
-      javascripts,
-      clean.javascripts
+      javascripts
     ),
     sass
   )