As a JS library author I pile and publish my source code in the following formats:

• monJS (/lib/index.js)
• ES (/es/index.js)
• UMD (/dist/index.js)

My unit tests cover my source code and I trust my bundling/piling tools to generate working bundles.

Then I read this enlightening post by React team where they explain that they run part of their unit tests against the bundled version of the library.

They introduced a test-build-prod which runs Jest with a special configuration file which replace the original import declarations in the test to point the bundled files using Jest's moduleNameMapper option.

It's cool but a bit overwhelming to replicate in my tiny open source side projects.

Before I give this a try, is there any other tool or more portable solution I should consider to run the same test I run on my source code against the piled bundles?

As a JS library author I pile and publish my source code in the following formats:

• monJS (/lib/index.js)
• ES (/es/index.js)
• UMD (/dist/index.js)

My unit tests cover my source code and I trust my bundling/piling tools to generate working bundles.

Then I read this enlightening post by React team where they explain that they run part of their unit tests against the bundled version of the library.

They introduced a test-build-prod which runs Jest with a special configuration file which replace the original import declarations in the test to point the bundled files using Jest's moduleNameMapper option.

It's cool but a bit overwhelming to replicate in my tiny open source side projects.

Before I give this a try, is there any other tool or more portable solution I should consider to run the same test I run on my source code against the piled bundles?

• javascript
• unit-testing
• jestjs

1 Answer 1

I'll share the solution I finally went with, which is the same adopted by React team but on small scale.

I added a special npm script to run my unit tests against each piled bundle with a different Jest configuration for each bundle:

{
    "test:bundles": "jest --config ./jest/es.config.js && jest --config ./jest/lib.config.js && jest --config ./jest/dist.config.js"
}

Each Jest configuration file extends default Jest configuration and declares a moduleNameMapper object plus a rootDir property like:

// jest/lib.config.js
const pkg = require('../package.json');

module.exports = Object.assign({}, pkg.jest, {
  rootDir: '../',
  moduleNameMapper: {
    '/src/index$': '<rootDir>/lib/index', // path of "CommonJS" bundle
  },
});

moduleNameMapper will make sure that the same tests used for source code will run against an exported bundle since Jest will transform the import statements on runtime like:

import myLibrary from './src/index';
// transformed into:
import myLibrary from '../lib/index';

The only point to pay attention to is making tests import statements needing remapping easily recognizable by moduleNameMapper regex.

If import from './index' is not unique enough in test the files, it might be rewritten as import from '../src/index'.