CommonJS (`require`)

CommonJS (require, module.exports) powered Node for years - you still maintain it in legacy packages, config loaders, and .cjs shims while new services standardize on ESM.

Recipe

// config.cjs
const path = require('node:path');
 
module.exports = {
  port: Number(process.env.PORT ?? 3000),
  root: __dirname,
  resolve: (p) => path.join(__dirname, p),
};

const config = require('./config.cjs');

When to reach for this:

Maintaining older npm dependencies that export CJS only
Jest configuration files still on require in brownfield repos
.cjs tooling hooks (jest.config.cjs, prettier.config.cjs)
createRequire bridges from ESM entry points

Working Example

// circular-a.cjs
const b = require('./circular-b.cjs');
module.exports = {
  name: 'A',
  bName: () => b.name,
};

// circular-b.cjs
const a = require('./circular-a.cjs');
module.exports = {
  name: 'B',
  aName: () => a.name,
};

// main.cjs
const a = require('./circular-a.cjs');
console.log(a.name, a.bName()); // A B - partial exports during circular init

// esm-dirname-polyfill.cjs - pattern ESM importers avoid by using import.meta.url
const { fileURLToPath } = require('node:url');
// Only needed when teaching CJS vs ESM differences

What this demonstrates:

require returns module.exports - assignments replace the whole export object
Circular requires return partial module.exports until modules finish initializing
__dirname is the directory of the current CJS file - no ESM equivalent without import.meta.url
Cache: second require('./same') returns the same object reference

Deep Dive

How It Works

Node resolves the path, wraps the file in a function, and caches module.exports in require.cache.
exports.foo = 1 is sugar for module.exports.foo until you reassign module.exports = ....
Builtin modules load first - require('node:fs') does not hit disk.
JSON can be require('./x.json') in CJS - ESM needs readFile + JSON.parse or import assertions patterns.

exports Patterns

Pattern	Result
`module.exports = fn`	Default export equivalent
`exports.a = 1`	Named property on exports
`module.exports = { a, b }`	Single object export

TypeScript Notes

// consume CJS from TS with esModuleInterop
import config from './config.cjs';
// or
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);
const cfg = require('./config.cjs') as { port: number };

Gotchas

Reassigning exports after module.exports = - breaks. Fix: only mutate module.exports once.
Circular dependency surprises - undefined imports mid-init. Fix: lazy require inside functions or refactor shared module.
Assuming __dirname in ESM - ReferenceError. Fix: import.meta.url + fileURLToPath.
Mutating required singletons - all importers see changes. Fix: treat exports as shared state intentionally or freeze.
Deep ../../../ paths - fragile moves. Fix: migrate to ESM with exports map or path aliases in tsconfig.

Alternatives

Alternative	Use When	Don't Use When
ESM `import`	All new application code	Unsupported legacy test runner without config
`createRequire`	ESM app needs one CJS dep	Entire package can be ESM
Dynamic `import()`	Async CJS interop from ESM	Sync config load at top of CJS
JSON module `import`	Static config in ESM	Need hot-reload without restart

FAQs

Is require deprecated?

Not removed, but ESM is the forward path. CommonJS remains for compatibility and .cjs config.

Can I use require in ESM?

Not directly. Use createRequire(import.meta.url) for exceptions.

What is require.cache?

Object keyed by resolved paths - delete entries for rare hot-reload in dev tools.

How do circular deps resolve?

Node returns incomplete module.exports until the module body finishes - design to avoid cycles.

Does require work with TypeScript files?

Not natively - compile to JS first or use tsx/ts-node loaders in dev.

What is .cjs extension?

Forces CommonJS parsing when package "type": "module".

Can I require ESM modules?

Generally no from CJS - ESM is async. Import ESM from ESM or use dynamic import bridge.

How do I export a function in CJS?

module.exports = function myFn() {} or module.exports = { myFn }.

Is module.exports === exports?

Initially exports references module.exports. Reassigning module.exports breaks the alias.

Why do some packages ship dual CJS/ESM?

Library authors support both ecosystems via exports conditions - consumers pick via import style.

Should new NestJS 11 apps use CJS?

Nest supports both; new projects increasingly use ESM with SWC - follow team ADR.

How do I migrate require to import?

Rename to .cjs only where needed, convert leaf modules first, then use createRequire temporarily at boundaries.

ES Modules (import) - the forward path
CJS ↔ ESM Interop - bridging strategies
package.json type & exports - package boundaries
Modules Basics - side-by-side intro

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.

CommonJS (`require`)

CommonJS (require, module.exports) powered Node for years - you still maintain it in legacy packages, config loaders, and .cjs shims while new services standardize on ESM.

Recipe

// config.cjs
const path = require('node:path');
 
module.exports = {
  port: Number(process.env.PORT ?? 3000),
  root: __dirname,
  resolve: (p) => path.join(__dirname, p),
};

const config = require('./config.cjs');

When to reach for this:

Maintaining older npm dependencies that export CJS only
Jest configuration files still on require in brownfield repos
.cjs tooling hooks (jest.config.cjs, prettier.config.cjs)
createRequire bridges from ESM entry points

Working Example

// circular-a.cjs
const b = require('./circular-b.cjs');
module.exports = {
  name: 'A',
  bName: () => b.name,
};

// circular-b.cjs
const a = require('./circular-a.cjs');
module.exports = {
  name: 'B',
  aName: () => a.name,
};

// main.cjs
const a = require('./circular-a.cjs');
console.log(a.name, a.bName()); // A B - partial exports during circular init

// esm-dirname-polyfill.cjs - pattern ESM importers avoid by using import.meta.url
const { fileURLToPath } = require('node:url');
// Only needed when teaching CJS vs ESM differences

What this demonstrates:

require returns module.exports - assignments replace the whole export object
Circular requires return partial module.exports until modules finish initializing
__dirname is the directory of the current CJS file - no ESM equivalent without import.meta.url
Cache: second require('./same') returns the same object reference

Deep Dive

How It Works

Node resolves the path, wraps the file in a function, and caches module.exports in require.cache.
exports.foo = 1 is sugar for module.exports.foo until you reassign module.exports = ....
Builtin modules load first - require('node:fs') does not hit disk.
JSON can be require('./x.json') in CJS - ESM needs readFile + JSON.parse or import assertions patterns.

exports Patterns

Pattern	Result
`module.exports = fn`	Default export equivalent
`exports.a = 1`	Named property on exports
`module.exports = { a, b }`	Single object export

TypeScript Notes

// consume CJS from TS with esModuleInterop
import config from './config.cjs';
// or
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);
const cfg = require('./config.cjs') as { port: number };

Gotchas

Reassigning exports after module.exports = - breaks. Fix: only mutate module.exports once.
Circular dependency surprises - undefined imports mid-init. Fix: lazy require inside functions or refactor shared module.
Assuming __dirname in ESM - ReferenceError. Fix: import.meta.url + fileURLToPath.
Mutating required singletons - all importers see changes. Fix: treat exports as shared state intentionally or freeze.
Deep ../../../ paths - fragile moves. Fix: migrate to ESM with exports map or path aliases in tsconfig.

Alternatives

Alternative	Use When	Don't Use When
ESM `import`	All new application code	Unsupported legacy test runner without config
`createRequire`	ESM app needs one CJS dep	Entire package can be ESM
Dynamic `import()`	Async CJS interop from ESM	Sync config load at top of CJS
JSON module `import`	Static config in ESM	Need hot-reload without restart

FAQs

Is require deprecated?

Not removed, but ESM is the forward path. CommonJS remains for compatibility and .cjs config.

Can I use require in ESM?

Not directly. Use createRequire(import.meta.url) for exceptions.

What is require.cache?

Object keyed by resolved paths - delete entries for rare hot-reload in dev tools.

How do circular deps resolve?

Node returns incomplete module.exports until the module body finishes - design to avoid cycles.

Does require work with TypeScript files?

Not natively - compile to JS first or use tsx/ts-node loaders in dev.

What is .cjs extension?

Forces CommonJS parsing when package "type": "module".

Can I require ESM modules?

Generally no from CJS - ESM is async. Import ESM from ESM or use dynamic import bridge.

How do I export a function in CJS?

module.exports = function myFn() {} or module.exports = { myFn }.

Is module.exports === exports?

Initially exports references module.exports. Reassigning module.exports breaks the alias.

Why do some packages ship dual CJS/ESM?

Library authors support both ecosystems via exports conditions - consumers pick via import style.

Should new NestJS 11 apps use CJS?

Nest supports both; new projects increasingly use ESM with SWC - follow team ADR.

How do I migrate require to import?

Rename to .cjs only where needed, convert leaf modules first, then use createRequire temporarily at boundaries.

ES Modules (import) - the forward path
CJS ↔ ESM Interop - bridging strategies
package.json type & exports - package boundaries
Modules Basics - side-by-side intro

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.

CommonJS (require)

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CommonJS (require)

CommonJS (`require`)

CommonJS (`require`)