ESM vs CommonJS

Node supports two JavaScript module systems. ESM uses import and export; CommonJS uses require and module.exports. A package can support both, but it must state how each consumer reaches the correct file.

Identify the format

Signal	Meaning
.mjs	Always ESM
.cjs	Always CommonJS
.js with "type": "module"	ESM
.js without that package type	CommonJS by default

Avoid guessing from syntax alone. Node resolves a .js file from its extension and the nearest package boundary.

Publish both formats

{
  "type": "module",
  "exports": {
    ".": {
      "import": "./dist/index.js",
      "require": "./dist/index.cjs"
    }
  }
}

Use an exports map rather than asking consumers to reach into dist. It gives each module system an intentional entry point and prevents accidental private imports becoming a public API.

Interoperability rules

• Keep the package API identical across formats where possible.
• Test both import and require from a consumer fixture.
• Do not publish TypeScript source as the runtime entry point.
• Use .cjs for CommonJS output when the package itself declares type: module.

Keep Reading

• Package Entry Points
• Type Declarations