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# Oh CommonJS! Why are you mESMing with me?

Tired of ESM and CommonJS clashes? The JavaScript ecosystem is moving on. Embrace native ECMAScript Modules to slash bundle sizes and eliminate technical debt.

- **Speakers:** [Jolo](https://www.wearedevelopers.com/@jolo)
- **Event:** World Congress 2024
- **Published:** August 20, 2024
- **Duration:** 20:23
- **URL:** https://www.wearedevelopers.com/videos/1059-oh-commonjs-why-are-you-mesming-with-me

## Summary

Upgrading routine dependencies can easily break existing CI pipelines when modern ESM-only packages clash with legacy CommonJS tools. This familiar frustration highlights the growing architectural rift in the JavaScript ecosystem between synchronous CommonJS (`require()`) and universally standardized ECMAScript Modules (`import()`). Navigating this divide is essential for maintaining scalable codebases without constantly managing fragile, experimental execution flags.

Although isolated CommonJS scripts might initially appear to execute faster, ECMAScript Modules deliver vastly superior performance in production workflows. Through aggressive tree-shaking supported by modern bundlers like esbuild, ESM aggressively discards unused code, yielding bundle sizes that are frequently half the size of their CommonJS equivalents. This drastically reduced resource footprint translates directly into improved execution speeds, making ESM the critical choice for optimizing cloud application architectures and serverless edge functions like AWS Lambda.

The industry trajectory points strictly toward an ESM-only future, naturally supported across browsers and modern server runtimes including Node.js, Deno, and Bun. Instead of retrofitting legacy constraints, developers should view breaking ecosystem changes as a crucial prompt to refactor. Replacing outdated tooling like Jest or Express with ESM-native alternatives such as Vitest and H3 effectively eliminates technical debt, permanently aligning the stack's architecture with modern development standards.

**Keywords:** javascript module systems, commonjs vs ESM, ecmascript modules, NPM dependency upgrades, jest testing framework, esbuild tree shaking, bundle size optimization, serverless edge functions, AWS Lambda performance, asynchronous module loading, legacy code refactoring, vitest migration, TypeScript module syntax, node.js runtimes, native browser modules

## Chapters

1. **Diagnosing unexpected token export errors in Jest test runners** (00:03) — Upgrading dependencies can trigger syntax errors when CommonJS test runners encounter modern ESM-only packages.
1. **Implementing module systems to scale server-side JavaScript execution** (05:58) — Splitting and reusing code becomes necessary as JavaScript execution shifts to complex server environments.
1. **Comparing CommonJS and ECMAScript module syntax and execution behaviors** (07:09) — Synchronous require statements in legacy CommonJS contrast with the asynchronous import standards of modern ECMAScript modules.
1. **Analyzing module system adoption trends across the package ecosystem** (09:17) — Despite most packages relying on older standards, the wider developer ecosystem shows a clear adoption trend toward native modules.
1. **Benchmarking execution speed and bundle size across module systems** (10:29) — Effective tree shaking makes bundled ECMAScript modules significantly smaller and faster to execute than CommonJS equivalents.
1. **Migrating legacy toolchains to native ECMAScript standard environments** (14:39) — Transitioning workflows to native modules and modern alternatives like Vitest eliminates ongoing legacy compatibility overhead.

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