Docs: Refactor Rollup recipe

Nothing extra is needed to use Rollup with gulp, so this refactors the
recipe to use Rollup's JavaScript API directly.

docs/recipes/README.md

@@ -18,5 +18,5 @@
 * [Output both a minified and non-minified version](minified-and-non-minified.md)
 * [Templating with Swig and YAML front-matter](templating-with-swig-and-yaml-front-matter.md)
 * [Run Grunt Tasks from Gulp](run-grunt-tasks-from-gulp.md)
-* [Rollup with rollup-stream](rollup-with-rollup-stream.md)
+* [Rollup](rollup.md)
 * [Run gulp task via cron job](cron-task.md)

docs/recipes/rollup.md

@@ -0,0 +1,71 @@
+# Rollup
+
+Rollup is a module bundler for JavaScript, and, while there are packages which aim to help use Rollup with gulp, like [`gulp-rollup`] and [`rollup-stream`](https://www.npmjs.com/package/rollup-stream), we can also use Rollup's JavaScript API directly.
+
+Let's say that we require the following features:
+
+  - being able to use Node modules
+  - interoperability between CommonJS and ES6 modules
+  - Babel
+  - Uglify in production
+
+To achieve this, we need to install the Rollup and its plugins:
+
+```sh
+npm install --save-dev gulp rollup @rollup/plugin-node-resolve @rollup/plugin-commonjs @rollup/plugin-babel @babel/core @babel/preset-env rollup-plugin-uglify
+```
+
+Create a basic `babel.config.js`:
+
+```js
+module.exports = {
+  presets: ["@babel/preset-env"],
+};
+```
+
+Use [Rollup's JavaScript API](https://rollupjs.org/guide/en/#javascript-api) to create tasks for compiling and recompiling:
+
+```js
+const rollup = require("rollup");
+const nodeResolve = require("@rollup/plugin-node-resolve").default;
+const commonJs = require("@rollup/plugin-commonjs");
+const babel = require("@rollup/plugin-babel").default;
+const uglify = require("rollup-plugin-uglify").uglify;
+
+const isProd = process.env.NODE_ENV === "production";
+
+const inputOptions = {
+  input: "scripts/index.js",
+  plugins: [
+    nodeResolve(),
+    commonJs(),
+    babel({ babelHelpers: "bundled" }),
+    ...(isProd ? [uglify()] : []),
+  ],
+};
+
+const outputOptions = {
+  file: `dist/script.js`,
+  format: "iife",
+  sourcemap: !isProd,
+};
+
+async function compileScripts() {
+  const bundle = await rollup.rollup(inputOptions);
+  await bundle.write(outputOptions);
+}
+
+function compileAndWatchScripts() {
+  // this function already creates a build initially, hence the name
+  // of the task, so you don't need to run "compileScripts" first
+  const watcher = rollup.watch({
+    ...inputOptions,
+    output: outputOptions,
+  });
+  watcher.on("event", (event) => {
+    if (event.code === "END") {
+      console.log("Compiled scripts");
+    }
+  });
+}
+```