Are there any performance differences between using CJS and ESM in Bun

Performance Observations

1. Loading Speed: CJS tends to load faster than ESM in Bun, especially for larger applications. This is primarily due to CJS's synchronous nature, which allows for immediate execution of `require()` calls, whereas ESM requires loading the entire module graph or awaiting each import statement[6][8].

2. Benchmark Results: In various benchmarks conducted, CJS has shown to be significantly faster than ESM. For instance, one benchmark indicated that loading Babel with CommonJS was about 2.4 times faster than with ES modules, with a difference of approximately 85 milliseconds[6]. Another user noted that even when running minimal code, the CJS version consistently outperformed its ESM counterpart[1].

3. Garbage Collection: Performance regressions in Node.js version 18 highlighted issues where a considerable amount of CPU time was spent on garbage collection when using ESM, further impacting performance negatively compared to CJS[1].

4. Complexity of Imports: The complexity introduced by ESM's asynchronous loading can also lead to slower performance in certain scenarios, especially when dealing with nested imports or large dependency graphs[8].

5. Real-World Applications: Developers migrating from CJS to ESM have reported noticeable slowdowns in their applications, suggesting that while ESM is the future standard for JavaScript modules, its current implementation may not yet match the performance efficiency of CJS in Bun and other environments[2][6].

Conclusion

While Bun supports both module systems and allows developers to mix them seamlessly, the current performance landscape indicates that CJS generally offers better loading speeds and efficiency compared to ESM. However, as ESM continues to evolve and become more optimized, these differences may diminish over time. For now, developers may choose based on their specific use cases and performance needs.