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Using Jest with Parcel Bundler

December 31, 2018

4 min read

#What is Parcel?

Parcel is a "zero-configuration" web application bundler. Its a very convenient alternative to a tool called Webpack. Using a bundler for your web application provides a lot of useful features like Hot Module Reloading, ES6 Transpilation, splitting your JavaScript into multiple files, and more. This article talks a bit about why to use webpack specifically, but is applicaple to most bundlers.

#What is Jest

Jest is a testing framework that works out of the box as well. It provides the ability to automatically run tests, a fleshed out assertion library, a powerful mocking library and more. Jest also has an emphasis on "zero-configuration" like Parcel.

#Why Jest and Parcel Don't Play Nicely by Default

When attempting to use multiple "zero-configuration" libraries together, you tend to run into edge cases that wind up requiring configuration after all.

By default, Jest is not able to handle transpilation of code. That task is handled by a tool like Babel. With a standard Webpack bundled project, you would manually configure Babel with a .babelrc file. Then you would allow Jest to use that configuration file by installing the NPM Package babel-jest. This package will automatically reference a root level .babelrc file.

However, because we're using Parcel, transpilation of our ES6 code is already handled for us and we have no Babel configuration.

#Remedying the Lack of Babel Situation

We need to install the following NPM packages to get a development environment with Parcel and Jest up and running.

  • parcel - our web application bundler
  • jest - our testing framework
  • babel-jest - our testing transpilation plugin
  • @babel/core - our transpilation library
  • @babel/preset-env - a set of common presets for transpilation

These packages can all be installed as developer dependencies with the following command.

npm i --save-dev parcel jest babel-jest @babel/core @babel/preset-env

Then in the root level of our project we need to create a .babelrc file with the following contents.

"presets": ["@babel/preset-env"]

To run your development, production, and test commands the following scripts in your package.json file can be used as a template.

"scripts": {
"start": "parcel index.html",
"build": "parcel build index.html",
"test": "jest --watch"