Getting Started

Detox - Testing React Native - Interview with Mykola Solopii

Before getting started, make sure you are using a recent version of Node. You should use at least the most current LTS (long-term support) version. The configuration of the book has been written with the LTS Node features in mind. You should have node and npm commands available at your terminal. Yarn is a good alternative to npm and works for the tutorial as well.

It's possible to get a more controlled environment by using a solution such as Docker, Vagrant or nvm. Vagrant comes with a performance penalty as it relies on a virtual machine. Vagrant is valuable in a team: each developer can have the same environment that is usually close to production.

The completed configuration is available at GitHub.

Setting Up the Project#

To get a starting point, you should create a directory for the project and set up a package.json there. npm uses that to manage project dependencies. Here are the basic commands:

mkdir webpack-demo
cd webpack-demo
npm init -y # -y generates *package.json*, skip for more control

You can tweak the generated package.json manually to make further changes to it even though a part of the operations modify the file automatically for you. The official documentation explains package.json options in more detail.

You can set those npm init defaults at ~/.npmrc.
This is an excellent chance to set up version control using Git. You can create a commit per step and tag per chapter, so it's easier to move back and forth if you want.
The book examples have been formatted using Prettier with "trailingComma": "es5", and "printWidth": 68 options enabled to make the diffs clean and fit the page.

Installing Webpack#

Even though webpack can be installed globally (npm install webpack -g), it's a good idea to maintain it as a dependency of your project to avoid issues, as then you have control over the exact version you are running. The approach works nicely in Continuous Integration (CI) setups as well. A CI system can install your local dependencies, compile your project using them, and then push the result to a server.

To add webpack to the project, execute:

npm install webpack webpack-cli --save-dev # -D to type less

You should see webpack at your package.json devDependencies section after this. In addition to installing the package locally below the node_modules directory, npm also generates an entry for the executable.

You can use --save and --save-dev to separate application and development dependencies. The former installs and writes to package.json dependencies field whereas the latter writes to devDependencies instead.
webpack-cli comes with additional functionality including init and migrate commands that allow you to create new webpack configuration fast and update from an older version to a newer one.

Executing Webpack#

You can display the exact path of the executables using npm bin. Most likely it points at ./node_modules/.bin. Try running webpack from there through the terminal using node_modules/.bin/webpack or a similar command.

After running, you should see a version, a link to the command line interface guide and an extensive list of options. Most aren't used in this project, but it's good to know that this tool is packed with functionality if nothing else.

$ node_modules/.bin/webpack
Hash: 6736210d3313db05db58
Version: webpack 4.1.1
Time: 88ms
Built at: 3/16/2018 3:35:07 PM

WARNING in configuration
The 'mode' option has not been set. Set 'mode' option to 'development' or 'production' to enable defaults for this environment.

ERROR in Entry module not found: Error: Can't resolve './src' in '.../webpack-demo'

The output tells that webpack cannot find the source to compile. It's also missing a mode parameter to apply development or production specific defaults.

To get a quick idea of webpack output, we should fix both:

  1. Set up src/index.js so that it contains console.log("Hello world");.
  2. Execute node_modules/.bin/webpack --mode development. Webpack will discover the source file by Node convention.
  3. Examine dist/main.js. You should see webpack bootstrap code that begins executing the code. Below the bootstrap, you should find something familiar.
Try also --mode production and compare the output.

Setting Up Assets#

To make the build more involved, we can add another module to the project and start developing a small application:


export default (text = "Hello world") => {
  const element = document.createElement("div");

  element.innerHTML = text;

  return element;

We also have to modify the original file to import the new file and render the application through the DOM:


import component from "./component";


Examine the output after building (node_modules/.bin/webpack --mode development). You should see both modules in the bundle that webpack wrote to the dist directory.

To make the output clearer to examine, pass --devtool false parameter to webpack. Webpack will generate eval based source maps by default and doing this will disable the behavior. See the Source Maps chapter for more information.

One problem remains, though. How can we test the application in the browser?

Configuring html-webpack-plugin#

The problem can be solved by writing an index.html file that points to the generated file. Instead of doing that on our own, we can use a plugin and webpack configuration to do this.

To get started, install html-webpack-plugin:

npm install html-webpack-plugin --save-dev

To connect the plugin with webpack, set up configuration as below:


const HtmlWebpackPlugin = require("html-webpack-plugin");

module.exports = {
  plugins: [
    new HtmlWebpackPlugin({
      title: "Webpack demo",

Now that the configuration is done, you should try the following:

  1. Build the project using node_modules/.bin/webpack --mode production. You can try the development mode too.
  2. Enter the build directory using cd dist.
  3. Run the server using serve (npm i serve -g) or a similar command.
  4. Examine the result through a web browser. You should see something familiar there.

Hello world
Hello world

Trailing commas are used in the book examples on purpose as it gives cleaner diffs for the code examples.

Examining the Output#

If you execute node_modules/.bin/webpack --mode production, you should see output:

Hash: aafe36ba210b0fbb7073
Version: webpack 4.1.1
Time: 338ms
Built at: 3/16/2018 3:40:14 PM
     Asset       Size  Chunks             Chunk Names
   main.js  679 bytes       0  [emitted]  main
index.html  181 bytes          [emitted]
Entrypoint main = main.js
   [0] ./src/index.js + 1 modules 219 bytes {0} [built]
       | ./src/index.js 77 bytes [built]
       | ./src/component.js 142 bytes [built]
Child html-webpack-plugin for "index.html":
     1 asset
    Entrypoint undefined = index.html
       [0] (webpack)/buildin/module.js 519 bytes {0} [built]
       [1] (webpack)/buildin/global.js 509 bytes {0} [built]
        + 2 hidden modules

The output tells a lot:

  • Hash: aafe36ba210b0fbb7073 - The hash of the build. You can use this to invalidate assets through [hash] placeholder. Hashing is discussed in detail in the Adding Hashes to Filenames chapter.
  • Version: webpack 4.1.1 - Webpack version.
  • Time: 338ms - Time it took to execute the build.
  • main.js 679 bytes 0 [emitted] main - Name of the generated asset, size, the IDs of the chunks into which it's related, status information telling how it was generated, the name of the chunk.
  • index.html 181 bytes [emitted] - Another generated asset that was emitted by the process.
  • [0] ./src/index.js + 1 modules 219 bytes {0} [built] - The ID of the entry asset, name, size, entry chunk ID, the way it was generated.
  • Child html-webpack-plugin for "index.html": - This is plugin-related output. In this case html-webpack-plugin is creating this output on its own.

Examine the output below the dist/ directory. If you look closely, you can see the same IDs within the source.

In addition to a configuration object, webpack accepts an array of configurations. You can also return a Promise and eventually resolve to a configuration for example.
If you want a light alternative to html-webpack-plugin, see mini-html-webpack-plugin. It does less but it's also simpler to understand.

Adding a Build Shortcut#

Given executing node_modules/.bin/webpack is verbose, you should do something about it. Adjust package.json to run tasks as below:


"scripts": {
  "build": "webpack --mode production"

Run npm run build to see the same output as before. npm adds node_modules/.bin temporarily to the path enabling this. As a result, rather than having to write "build": "node_modules/.bin/webpack", you can do "build": "webpack".

You can execute this kind of scripts through npm run and you can use npm run anywhere within your project. If you run the command as is, it gives you the listing of available scripts.

There are shortcuts like npm start and npm test. You can run these directly without npm run although that works too. For those in a hurry, you can use npm t to run your tests.
To go one step further, set up system level aliases using the alias command in your terminal configuration. You could map nrb to npm run build for instance.

HtmlWebpackPlugin Extensions#

Although you can replace HtmlWebpackPlugin template with your own, there are premade ones like html-webpack-template or html-webpack-template-pug.

There are also specific plugins that extend HtmlWebpackPlugin's functionality:


Even though you have managed to get webpack up and running, it does not do that much yet. Developing against it would be painful. Each time you wanted to check out the application, you would have to build it manually using npm run build and then refresh the browser. That's where webpack's more advanced features come in.

To recap:

  • It's a good idea to use a locally installed version of webpack over a globally installed one. This way you can be sure of what version you are using. The local dependency also works in a Continuous Integration environment.
  • Webpack provides a command line interface through the webpack-cli package. You can use it even without configuration, but any advanced usage requires configuration.
  • To write more complicated setups, you most likely have to write a separate webpack.config.js file.
  • HtmlWebpackPlugin can be used to generate an HTML entry point to your application. In the Multiple Pages chapter you will see how to generate multiple separate pages using it.
  • It's handy to use npm package.json scripts to manage webpack. You can use it as a light task runner and use system features outside of webpack.

In the next chapter, you will learn how to improve the developer experience by enabling automatic browser refresh.

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This book is available through Leanpub (digital), Amazon (paperback), and Kindle (digital). By purchasing the book you support the development of further content. A part of profit (~30%) goes to Tobias Koppers, the author of webpack.

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