Configuring Jest with React and Babel


Posted: 2019-08-26T11:38:00+05:30 | Source | More posts about babel es6 frontend javascript jest react testing unittesting
Jest is a really good frontend testing framework and works great with React and Babel out of the box, along with Enzyme for component testing. But, imports with React and Babel can often be filled with nasty imports. I wrote in a previous blog about how to make better more cleaner imports using some webpack tweaks.

But the problem appears when we try to write Jest and Enzyme tests with them. Because Babel can now longer understand and parse the imports. And without Babel parsing them and converting to ES5, jest cannot test the components. So we actually need a mix of Babel configuration and Jest configuration.

Note: This post assumes you already have jest, babel-jest and babel/plugin-transform-modules-commonjs packages installed using your favorite javascript package manager.

Basically, the workaround is first we need to resolve the cleaner imports into the absolute paths for the import using Jest configurations, and then use the Babel configurations to parse the rest code (without the modules) to ES5.

The configuration files look something like these:

babel.config.js

module.exports = api => {
const isTest = api.env('test');
if (isTest) {
return {
presets: [
[
'@babel/preset-env',
{
modules: false,
},
],
'@babel/preset-react',
],
plugins: [
"@babel/plugin-transform-modules-commonjs",
],
}
} else {
return {
presets: [
[
'@babel/preset-env',
{
modules: false,
},
],
'@babel/preset-react',
],
}
}
};


jest.config.js

module.exports = {
moduleNameMapper: {
'^~/(.*)$': '<rootDir>/path/to/jsRoot/$1'
}
}

So let's go through the code a little.

In babel.config.js, we make a check to see if the code is right now in test environment. This is helpful because
  1. Jest sets the environment to "test" when running a test so it is easily identifiable
  2. It ensures that the test configuration don't mess up with the non test configurations in our Webpack config (or any other configuration you are using)
So in this case, I am returning the exact same Babel configuration that I need in my Webpack config in non-test environment.

In the test configuration for Babel, we are using a plugin "@babel/plugin-transform-modules-commonjs". This is needed to parse all the non component imports like React, etc. along with parsing the components from ES6 to ES5 after jest does the path resolution. So it helps to convert the modules from ES6 to ES5.

Now, let's see the jest.config.js. The jest configuration allows us to do something called moduleNameMapper. This is a very useful configuration in many different usecases. It basically allows us to convert the module names or paths we use for module import to something that jest understands (or in our case, something that the Babel plugin can parse). 

So, the left hand part of the attribute contains a regular expression which matches the pattern we are using for imports. Since our imports look something like '~/path/from/jsRoot/Component', so the regular expression to capture all such imports is '^~/(.*)$'. Now, to convert them to absolute paths, we need to append '<rootDir>/path/to/jsRoot/' in front of the component path.

And, voila! That should allow Jest to properly parse, convert to ES5 and then test.

The best part? We can use the cleaner imports even in the .test.js files and this configuration will work perfectly with that too.