JavaScript Runtime Keywords

Modern JavaScript modules have different runtime globals depending on whether Node loads them as CommonJS or ESM. Know the boundary before copying code between build tools, scripts, and package entry points.

CommonJS values

console.log(__filename)
console.log(__dirname)
console.log(require.main)

These values are available in CommonJS modules. They are not automatically present in ESM files.

ESM equivalents

import path from 'node:path'
import { fileURLToPath } from 'node:url'

const filename = fileURLToPath(import.meta.url)
const dirname = path.dirname(filename)

Use import.meta.url as the starting point for resolving files relative to an ESM module. Prefer explicit Node built-in imports such as node:path so bundlers and readers can identify runtime-only dependencies.

Keep code portable

• Keep filesystem access behind a small module when browser builds share source code.
• Do not use CommonJS globals in a package that declares type: module.
• Test generated ESM and CommonJS outputs independently when both are published.

Keep Reading

• JavaScript, CommonJS, and ESM Files
• Reusing JavaScript