ESM vs CommonJS — what's the difference?

ESM is statically analyzable, async, the standard. CJS is dynamic, sync, Node-historical. Modern code is ESM; CJS lives on for legacy compatibility. Mixing them is the source of many Node interop bugs.

6 min read·~12 min to think through

CommonJS (CJS) is the original Node module system: module.exports / require(). It's dynamic — require() is a function that runs at call time, returns whatever object the module decided to set. Synchronous — execution blocks until the module's code finishes.

ES Modules (ESM) is the standard: export / import. Static — imports/exports are declared at the top, parseable before execution. Asynchronous — modules can be loaded in parallel; top-level await works.

Why the distinction matters in practice:

Tree shaking works well with ESM (static export shape) and barely works with CJS.
Top-level await is ESM-only.
Dual packages (exports map in package.json) let a library ship both for compatibility, but the same module loaded twice (once CJS, once ESM) causes "instanceof" and singleton bugs.
Node interop — import of CJS in ESM works; require of ESM doesn't (synchronously). Use await import().
Browsers support ESM natively (<script type="module">); CJS never worked in browsers without a bundler.

Migration tips: prefer "type": "module" for new packages. Author in TS, emit ESM, expose CJS only as a fallback build via exports map.

Code

Follow-up questions

•What's a 'dual package hazard' and how do you avoid it?
•Why can't you `require()` an ESM module synchronously?
•How does Node decide whether a `.js` file is ESM or CJS?

Common mistakes

•Mixing default and named imports of a CJS package in TypeScript and being surprised by `interop=true` output.
•Top-level await in a file Node treats as CJS — throws.
•Two copies of the same package (CJS + ESM) in a bundle, breaking instanceof checks.

Performance considerations

•ESM enables aggressive tree shaking. CJS bundles ship more bytes.

Edge cases

•JSON imports need an import attribute: `import data from './x.json' with { type: 'json' }`.
•Conditional exports can break older Node — test with the lowest supported version.

Real-world examples

•TypeScript 5.0+ ships ESM-first; chalk@5 went ESM-only and broke many CJS consumers — a public migration lesson.

Senior engineer discussion

Senior signal: discuss exports/conditions, the dual package hazard, how Node's module resolution algorithm works, and the cost of maintaining a CJS fallback.