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

app/assets/javascripts/modules/all.mjs

@@ -2,10 +2,10 @@
 // a bit like `app/__init__` in the Flask app.
 //
 // When processed by a bundler, this is turned into a Immediately Invoked Function Expression (IIFE)
-// and saved as `all.js` in the same folder as this file. The IIFE format allows it to run in
+// The IIFE format allows it to run in browsers that don't support JS Modules.
-// browsers that don't support JS Modules.
 //
 // Exported items will be added to the window.GOVUK namespace.
+// For example, `export { Frontend }` will assign `Frontend` to `window.Frontend`
 import Header from 'govuk-frontend/components/header/header';
 
 // Copy of the initAll function from https://github.com/alphagov/govuk-frontend/blob/v2.13.0/src/all.js

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');
@@ -54,9 +54,18 @@ const copy = {
 };
 
 
-const bundleJavaScriptModules = async function () {
+
-  const bundle = await rollup.rollup({
+
-    input: paths.src + 'javascripts/modules/all.mjs',
+const javascripts = () => {
+  // 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({
@@ -68,20 +77,24 @@ const bundleJavaScriptModules = async function () {
             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'
-  });
+      }
-};
+    ))
+    // 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'
+    ]));
 
-
+  // JS local to this application
-const javascripts = () => {
+  const local = src([
-  return src([
     paths.toolkit + 'javascripts/govuk/modules.js',
     paths.toolkit + 'javascripts/govuk/show-hide-content.js',
     paths.src + 'javascripts/govuk/cookie-functions.js',
@@ -108,16 +121,11 @@ const javascripts = () => {
   .pipe(plugins.prettyerror())
   .pipe(plugins.babel({
     presets: ['@babel/preset-env']
-    }))
+  }));
-    .pipe(plugins.addSrc.prepend([
+
-      paths.npm + 'hogan.js/dist/hogan-3.0.2.js',
+  // return single stream of all vinyl objects piped from the end of the vendored stream, then
-      paths.npm + 'jquery/dist/jquery.min.js',
+  // those from the end of the local stream
-      paths.npm + 'query-command-supported/dist/queryCommandSupported.min.js',
+  return streamqueue({ objectMode: true }, vendored, local)
-      paths.npm + 'diff-dom/diffDOM.js',
-      paths.npm + 'timeago/jquery.timeago.js',
-      paths.npm + 'textarea-caret/index.js',
-      paths.src + 'javascripts/modules/all.js'
-    ]))
     .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',
+        paths.src + 'javascripts/**/*.js'
-        { ignore: paths.src + 'javascripts/modules/*.js' } // ignore bundler boilerplate 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
-      javascripts,
-      clean.javascripts
     ),
     sass
   )

package.json

@@ -30,6 +30,7 @@
     "gulp-add-src": "1.0.0",
     "gulp-babel": "8.0.0",
     "gulp-base64-inline": "1.0.4",
+    "gulp-better-rollup": "4.0.1",
     "gulp-concat": "2.6.1",
     "gulp-include": "2.3.1",
     "gulp-sass": "4.0.2",
@@ -38,6 +39,7 @@
     "jquery": "3.4.1",
     "query-command-supported": "1.0.0",
     "rollup": "1.23.1",
+    "streamqueue": "1.1.2",
     "textarea-caret": "3.1.0",
     "timeago": "1.6.5"
   },