How to use webpack with babel-loader and flow

Question

I'm struggling with what I feel shouldn't be too hard of a setup. I want these three technologies to work together:

• webpack to bundle code
• babel so that I'm able to write modern JavaScript
• flow because type checking is helpful just like tests and linter

I've already gone trough several setups of this but none of the articles I've found on the web appears to be helpful.

---

I've defined 3 scripts in my package.json, run, flow and build. Typechecking with yarn run flow works perfectly, and so does executing the script trough babel-node using yarn run start.

But when I execute yarn run build the following error appears trough webpack:

$ ./node_modules/webpack/bin/webpack.js
Hash: 207d42dac5784520fc99
Version: webpack 3.10.0
Time: 49ms
    Asset     Size  Chunks             Chunk Names
bundle.js  2.65 kB       0  [emitted]  main
   [0] ./src/main.js 181 bytes {0} [built] [failed] [1 error]

ERROR in ./src/main.js
Module parse failed: Unexpected token (3:5)
You may need an appropriate loader to handle this file type.
| // @flow
|
| type Foo = {
|   foo: string,
| };
error Command failed with exit code 2.

It appears to me that the type annotations don't get removed correctly at the right point. Sadly this also happend if I specify the babel options in webpack directly as opposed to the .babelrc.

This currently defeats me in bundling up a bunch of .js files whilst using flow and while I found several plugins that supposedly strip flow annotations simply using the flow preset is what flowtype.org appears to recommend.

---

For reproducibillity, my project files look like this:

package.json:

{
  …
  "dependencies": {},
  "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-core": "^6.26.0",
    "babel-eslint": "^8.0.3",
    "babel-loader": "^7.1.2",
    "babel-preset-env": "^1.6.1",
    "babel-preset-flow": "^6.23.0",
    "flow-bin": "^0.60.1",
    "webpack": "^3.9.1"
  },
  "scripts": {
    "build": "./node_modules/webpack/bin/webpack.js",
    "start": "./node_modules/babel-cli/bin/babel-node.js src/main.js",
    "flow": "./node_modules/flow-bin/cli.js"
  }
}

.flowconfig:

[include]
./src

[ignore]

[libs]

[lints]

[options]

.babelrc:

 {
   "presets": ["env", "flow"]
 }

webpack.config.js:

const path = require('path');
const webpack = require('webpack');

module.exports = {
  entry: './src/main.js',
  output: {
    path: __dirname,
    filename: 'bundle.js',
  },
  resolve: {
    modules: [
      path.resolve('./src/'),
      'node_modules',
    ],
    extensions: ['.js'],
  },
  module: {
    rules: [
      {
        test: '/.js$/',
        loader: 'babel-loader',
      },
    ],
  },
};

src/main.js:

// @flow

type Foo = {
  foo: string,
};

const defaultFoo: Foo = {
  foo: 'bar',
};

console.log(defaultFoo);

Answer 1

Download the relevant presets and add them to the webpack.config.js as follows:

module: {
  rules: [
    {
      test: /\.js$/,
      exclude: /(node_modules|bower_components)/,
      use: {
        loader: 'babel-loader',
        options: {
          presets: ['es2015', 'stage-0', 'react']
        }
      }
    }
  ]
}