update tree shaking guide for webpack 4

Author: quantizor

src/content/guides/tree-shaking.md

@@ -8,6 +8,7 @@ contributors:
   - avant1
   - MijaelWatts
   - dmitriid
+  - probablyup
 related:
   - title: Tree shaking with webpack 2, TypeScript and Babel
     url: https://alexjoverm.github.io/2017/03/06/Tree-shaking-with-Webpack-2-TypeScript-and-Babel/
@@ -23,7 +24,7 @@ related:
 
 _Tree shaking_ is a term commonly used in the JavaScript context for dead-code elimination. It relies on the [static structure](http://exploringjs.com/es6/ch_modules.html#static-module-structure) of ES2015 module syntax, i.e. [`import`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) and [`export`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export). The name and concept have been popularized by the ES2015 module bundler [rollup](https://github.com/rollup/rollup).
 
-The webpack 2 release came with built-in support for ES2015 modules (alias _harmony modules_) as well as unused module export detection.
+The webpack 2 release came with built-in support for ES2015 modules (alias _harmony modules_) as well as unused module export detection. The new webpack 4 release expands on this capability with a way to provide hints to the compiler via the `"sideEffects"` `package.json` flag to denote which files in your project are "pure" and therefore safe to prune if unused.
 
 T> The remainder of this guide will stem from [Getting Started](/guides/getting-started). If you haven't read through that guide already, please do so now.
 
@@ -107,23 +108,46 @@ function cube(x) {
 Note the `unused harmony export square` comment above. If you look at the code below it, you'll notice that `square` is not being imported, however, it is still included in the bundle. We'll fix that in the next section.
 
 
-## Minify the Output
+## Mark the file as side-effect-free
+
+In a 100% ESM module world, identifying side effects is straightforward. However, we don't live in that reality just yet, so it's necessary to provide hints to webpack's compiler on the "pureness" of your code.
+
+The way this is accomplished is the `"sideEffects"` package.json property.
+
+```json
+{
+  "name": "your-project",
+  "sideEffects": false
+}
+```
 
-So we've cued up our "dead code" to be dropped by using the `import` and `export` syntax, but we still need to drop it from the bundle. To do that, we'll add a minifier that supports dead code removal -- the [`UglifyJSPlugin`](/plugins/uglifyjs-webpack-plugin) -- to our configuration...
+All the code noted above does not contain side effects, so we can simply mark the property as `false` to inform webpack that it can safely prune unused exports.
 
-Let's start by installing it:
+T> A "side effect" is defined as code that performs a special behavior when imported, other than simply exposing one or more exports. For instance: a polyfill which adjusts the global scope but does not provide an export or has one that you are not using in your codebase.
 
-``` bash
-npm install --save-dev uglifyjs-webpack-plugin
+If your code did have some side effects though, an array can be provided instead:
+
+```json
+{
+  "name": "your-project",
+  "sideEffects": [
+    "./src/some-side-effectful-file.js"
+  ]
+}
 ```
 
-And then adding it into our config:
+## Minify the Output
+
+So we've cued up our "dead code" to be dropped by using the `import` and `export` syntax, but we still need to drop it from the bundle. To do that, we'll use the `-p` (production) webpack compilation flag to enable the uglifyjs minification plugin.
+
+T> Note that the `--optimize-minimize` flag can be used to insert the `UglifyJsPlugin` as well.
+
+As of webpack 4, this is also easily toggled via the `"mode"` config option, set to `"production"`.
 
 __webpack.config.js__
 
 ``` diff
 const path = require('path');
-+ const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
 
 module.exports = {
   entry: './src/index.js',
@@ -132,57 +156,21 @@ module.exports = {
     path: path.resolve(__dirname, 'dist')
 - }
 + },
-+ plugins: [
-+   new UglifyJSPlugin()
-+ ]
++ mode: "production"
 };
 ```
 
-T> Note that the `--optimize-minimize` flag can be used to insert the `UglifyJsPlugin` as well.
-
 With that squared away, we can run another `npm run build` and see if anything has changed.
 
 Notice anything different about `dist/bundle.js`? Clearly the whole bundle is now minified and mangled, but, if you look carefully, you won't see the `square` function included but will see a mangled version of the `cube` function (`function r(e){return e*e*e}n.a=r`). With minification and tree shaking our bundle is now a few bytes smaller! While that may not seem like much in this contrived example, tree shaking can yield a significant decrease in bundle size when working on larger applications with complex dependency trees.
 
 
-## Caveats
-
-Please note that webpack doesn't perform tree-shaking by itself. It relies on third party tools like [UglifyJS](/plugins/uglifyjs-webpack-plugin/) to perform actual dead code elimination. There are situations where tree-shaking may not be effective. For example, consider the following modules:
-
-__transforms.js__
-
-``` js
-import * as mylib from 'mylib';
-
-export const someVar = mylib.transform({
-  // ...
-});
-
-export const someOtherVar = mylib.transform({
-  // ...
-});
-```
-
-__index.js__
-
-``` js
-import { someVar } from './transforms.js';
-
-// Use `someVar`...
-```
-
-In the code above webpack cannot determine whether or not the call to `mylib.transform` triggers any side-effects. As a result, it errs on the safe side and leaves `someOtherVar` in the bundled code.
-
-In general, when a tool cannot guarantee that a particular code path doesn't lead to side-effects, this code may remain in the generated bundle even if you are sure it shouldn't. Common situations include invoking a function from a third-party module that webpack and/or the minifier cannot inspect, re-exporting functions imported from third-party modules, etc.
-
-The code used in this guide assumes you perform tree-shaking using UglifyJS plugin. However, there are other tools such as [webpack-rollup-loader](https://github.com/erikdesjardins/webpack-rollup-loader) that may produce different results depending on your setup.
-
-
 ## Conclusion
 
 So, what we've learned is that in order to take advantage of _tree shaking_, you must...
 
 - Use ES2015 module syntax (i.e. `import` and `export`).
+- Add a "sideEffects" entry to your project's `package.json` file.
 - Include a minifier that supports dead code removal (e.g. the `UglifyJSPlugin`).
 
 You can imagine your application as a tree. The source code and libraries you actually use represent the green, living leaves of the tree. Dead code represents the brown, dead leaves of the tree that are consumed by autumn. In order to get rid of the dead leaves, you have to shake the tree, causing them to fall.