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Convert MJS to CJS

Verified by Adam B.· reviewed by FE Editorial ·updated Aug 13, 2026
QUICK ANSWER
Is it possible to convert MJS to CJS?
Yes - MJS converts to CJS.

A .mjs file uses ES module syntax, while .cjs uses CommonJS. A bundler like esbuild or Babel rewrites import to require and export to module.exports automatically. The main blocker is top-level await, which CommonJS cannot represent.

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Tested on macOS, Windows & Linux
Last verified Sep 2026

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Why convert MJS to CJS?

People convert to .CJS to run code in older Node.js setups or tools that expect CommonJS, or to ship a require-compatible build alongside an ESM one. The logic stays the same; only the module wiring changes.

How to convert MJS to CJS

esbuild EASIEST

Run esbuild input.mjs --format=cjs --platform=node --outfile=output.cjs to emit CommonJS. Add --bundle if you also want dependencies inlined.

Babel OPEN-SOURCE

Add @babel/plugin-transform-modules-commonjs to your config, then run babel input.mjs -o output.cjs to rewrite imports to require.

Rollup FREE

Set output.format to cjs in rollup.config.js and build; Rollup emits a CommonJS bundle from your ESM entry.

About these formats

.MJS Node.js ES Module File

An .mjs file is JavaScript source code that Node.js treats as an ECMAScript module, meaning it uses import and export instead of CommonJS require. The extension exists so Node can tell ES…

Open .MJS details →
.CJS CJS File

A .CJS file is a data file used by specific software.

Open .CJS details →

Quality & what to watch

  • Top-level await has no CommonJS equivalent and will fail to convert.
  • Named exports become properties on module.exports, and interop with default exports can differ subtly.
  • Relative import paths and import.meta.url may need adjusting for the CommonJS output.

Frequently asked questions

Can I convert by hand?
For small files yes: swap import x from 'y' for const x = require('y') and export for module.exports.
Why did my build fail?
Most often top-level await, which CommonJS cannot express; move it inside an async function.
Do I need to rename to .cjs?
Yes, if your package.json has "type": "module"; the .cjs extension forces Node to treat it as CommonJS.
Which tool is fastest?
esbuild is the quickest for a single command; Babel and Rollup give finer control over the output.