ES6+ on legacy browsers

Mithril.js is written in ES5, but it's fully compatible with ES6 and later as well. All modern browsers do support it natively, up to and even including native module syntax. (They don't support Node's magic module resolution, so you can't use import * as _ from "lodash-es" or similar. They just support relative and URL paths.) And so you can feel free to use arrow functions for your closure components and classes for your class components.

But, if like many of us, you still need to support older browsers like Internet Explorer, you'll need to transpile that down to ES5, and this is what this page is all about, using Babel to make modern ES6+ code work on older browsers.

Setup

First, if you haven't already, make sure you have Node installed. It comes with npm pre-bundled, something we'll need soon.

Once you've got that downloaded, open a terminal and run these commands:

# Replace this with the actual path to your project. Quote it if it has spaces,
# and single-quote it if you're on Linux/Mac and it contains a `$$` anywhere.
cd "/path/to/your/project"

# If you have a `package.json` there already, skip this command.
npm init

Now, you can go one of a couple different routes:

• Use Babel standalone, with no bundler at all
• Use Babel and bundle with Webpack

Using Babel standalone

First, we need to install a couple dependencies we need.

• @babel/cli installs the core Babel logic as well as the babel command.
• @babel/preset-env helps Babel know what to transpile and how to transpile them.

npm install @babel/cli @babel/preset-env --save-dev

Now, create a .babelrc file and set up with @babel/preset-env.

{
  "presets": ["@babel/preset-env"],
  "sourceMaps": true
}

And finally, if you have very specific requirements on what you need to support, you may want to configure Browserslist so Babel (and other libraries) know what features to target.

By default, if you don't configure anything, Browserslist uses a fairly sensible query: > 0.5%, last 2 versions, Firefox ESR, not dead. Unless you have very specific circumstances that require you to change this, like if you need to support IE 8 with a lot of polyfills, don't bother with this step.

Whenever you want to compile your project, run this command, and everything will be compiled.

babel src --out-dir dist

You may find it convenient to use an npm script so you're not having to remember this and typing it out every time. Add a "build" field to the "scripts" object in your package.json:

{
  "scripts": {
    "build": "babel src --out-dir dist"
  }
}

And now, the command is a little easier to type and remember.

npm run build

Using Babel with Webpack

If you want to use Webpack to bundle, it's a few more steps to set up. First, we need to install all the dependencies we need for both Babel and Webpack.

• webpack is the core Webpack code and webpack-cli gives you the webpack command.
• @babel/core is the core Babel code, a peer dependency for babel-loader.
• babel-loader lets you teach Webpack how to use Babel to transpile your files.
• @babel/preset-env helps Babel know what to transpile and how to transpile them.

npm install webpack webpack-cli @babel/core babel-loader @babel/preset-env --save-dev

Now, create a .babelrc file and set up with @babel/preset-env.

{
  "presets": ["@babel/preset-env"],
  "sourceMaps": true
}

Next, if you have very specific requirements on what you need to support, you may want to configure Browserslist so Babel (and other libraries) know what features to target.

By default, if you don't configure anything, Browserslist uses a fairly sensible query: > 0.5%, last 2 versions, Firefox ESR, not dead. Unless you have very specific circumstances that require you to change this, like if you need to support IE 8 with a lot of polyfills, don't bother with this step.

And finally, set up Webpack by creating a file called webpack.config.js.

const path = require('path');

module.exports = {
  entry: path.resolve(__dirname, 'src/index.js'),
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'app.js',
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /\/node_modules\//,
        use: {
          loader: 'babel-loader',
        },
      },
    ],
  },
};

This configuration assumes the source code file for the application entry point is in src/index.js, and this will output the bundle to dist/app.js.

Now, to run the bundler, you just run this command:

webpack -d --watch

You may find it convenient to use an npm script so you're not having to remember this and typing it out every time. Add a "build" field to the "scripts" object in your package.json:

{
  "scripts": {
    "start": "webpack -d --watch"
  }
}

And now, the command is a little easier to type and remember.

npm start

For production builds, you'll want to minify your scripts. Luckily, this is also pretty easy: it's just running Webpack with a different option.

webpack -p

You may want to also add this to your npm scripts, so you can build it quickly and easily.

{
  "scripts": {
    "start": "webpack -d --watch",
    "build": "webpack -p"
  }
}

And then running this is a little easier to remember.

npm run build

And of course, you can do this in automatic production build scripts, too. Here's how it might look if you're using Heroku, for example:

{
  "scripts": {
    "start": "webpack -d --watch",
    "build": "webpack -p",
    "heroku-postbuild": "webpack -p"
  }
}