JavaScript, CommonJS, and ESM Files

File extensions tell Node which module loader should read a file. Use them deliberately when a package publishes both module formats or when a script must work independently of the nearest package.json.

Extension rules

Extension	Module format
.mjs	ESM in every package
.cjs	CommonJS in every package
.js	Determined by the nearest package.json type field

{
  "type": "module"
}

With this declaration, .js files are ESM. Use .cjs for CommonJS output such as a compatibility build or an older tooling configuration file.

Avoid ambiguous output

dist/index.js     ESM output
dist/index.cjs    CommonJS output
dist/index.d.ts   Type declarations

Connect these files through exports; do not ask consumers to infer a dist layout. A clear output layout also makes package tarball checks easier.

Keep Reading

• ESM vs CommonJS
• Package Entry Points