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What is the difference between "type": "module", .mjs, and .cjs?

Q: What is the difference between "type": "module", .mjs, and .cjs?

**TL;DR.** Node decides a file's module system **per file**: .mjs is **always ESM**, .cjs is **always CommonJS**, and a plain .js follows the **nearest package.json "type" ** field — "module" means ESM, while "commonjs" or absent means CJS. The explicit extensions override the package default. .mjs ESM .cjs

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How Node decides whether a file is ESM or CommonJS.

nodejsesmcommonjsmodules

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Mixing systems in one package

// package.json
{ "type": "module" }      // .js files here are ESM

// index.js  → ESM
import { helper } from './helper.js';

// legacy.cjs → CommonJS even though package is "module"
const old = require('old-lib');
module.exports = { old };

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