Jest, ESM, and the Meta Tech Conundrum: A Cautionary Tale

You know that moment when you’re deep in your TypeScript project, everything’s going smooth, and then you import a simple CSV parsing library? Yeah, that’s when Jest decides to remind you about Facebook’s - sorry, Meta’s - opinions on JavaScript modules.

Here’s what happened in my project today: A simple import from csv-parse/lib/sync led me down a rabbit hole of Jest configuration hell. Why? Because Jest, in its infinite CommonJS wisdom, throws a tantrum when it sees modern ES Modules. Jest was built when CommonJS was the Node.js standard, and its adaptation to the ESM world has been slower than ideal.

// This innocent line of code
import { parse } from 'csv-parse/lib/sync'

// Led to this beauty of a config
moduleNameMapper: {
  '^csv-parse/lib/sync$': 'csv-parse/lib/sync.js'
},
transformIgnorePatterns: [
  'node_modules/(?!(csv-parse)/)'
]

Let’s talk about why this is absurd in 2024:

1. ES Modules are the future (or rather, the present) of JavaScript
2. Node.js has supported them for years
3. Most modern packages are moving to ESM
4. Yet Jest still lives in its CommonJS comfort zone

The real lesson? Meta’s tools are like they work when they work, but when they don’t, you’ll find yourself adding more and more configuration to handle basic features that should just work with other tools. Most of those tools, (Not you react!) were invented so that Facebook could solve their own issues or their on mess ups in the systems.

This will work with you untill it doesn’t, and when it doesn’t, Facebook’s move fast and break things hit you hard. While the scale and aura of Facebook makes you considering adapting these tools, you do that, add them into your project and then you are constantly praying the god that next update won’t break them.

Back in my android days, I fought with so much of such incidents that I don’t have enough appetite to consider Meta’s tools in my projects. Even in SourceSailor I had gone for vite instead of Jest. Only thing in recent years that changed my perspective was React, and Llama models. But they are exception, not the rule.

If you’re starting a new project, think twice before reaching for Meta’s toolbox. Sure, LLAMA models are cool, but development tools? They’re built for Meta’s problems, not yours.

Meanwhile, if you’re stuck with Jest in a work repository like I am, here’s your new best friend:

transformIgnorePatterns: ['node_modules/(?!(package-with-esm)/)']

Keep it handy. You’ll need it more often than you’d like.

I hope you may not need it, but if you do, I hope this helps you.

Till then

May the force be with you…