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This preset provides utilities for Jest and Hot Module Replacement in React projects.


npm install @swissquote/crafty-preset-babel --save
module.exports = {
  presets: [
    "@swissquote/crafty-preset-babel", // also works with crafty-preset-typescript
    "@swissquote/crafty-runner-webpack", // optional
  js: {
    app: {
      runner: "webpack",
      source: "js/app.js",
      hot: true, // Hot Module Replacement must be enabled for React Hot Loader to work
      react: true // React Hot Loader must be explicitly enabled in your bundle

Testing React components in Jest

This preset comes with Enzyme already configured for React 16.

This allows you to do shallow rendering of components, execute events and check the state of components.

Their documentation on testing components is exhaustive.

Check it out

Hot Module Replacement

When doing modern JavaScript development, the usual process is Write code, Compile, Refresh the browser.

  • crafty watch removes the Compile step because it's run automatically.
  • Hot Module Replacement (HMR) was created to remove the Refresh the browser part.

More precisely, when doing a build with Webpack, in development mode, a Websocket client is added to the build and a small HTTP server is started. When the page is loaded, each bundle will establish a Websocket connection to the server.

When you change a line of code, the server will rebuild them and send a notification through Websockets to the browser, the browser will then download the patch and apply the code change.

With React components, it will even re-render them without losing the current state.

Here's an example :

React Hot Module Replacement example

To enable HMR on your react application, you must set hot: true and react: true on your bundle in crafty.config.js.

Then you must mark your root component as hot-exported :

import React from "react";
import { hot } from "react-hot-loader";

const App = () => <div>Hello World!</div>;

export default hot(module)(App);

Only the root component needs this wrapping, the child components don't need it.


If you're using TypeScript you have one more step to make

npm install --save @types/webpack-env

Read more about Hot Module Replacement