CommonJS vs ES Modules: what is the difference?

Definition:

CommonJS and ES Modules are two systems for dividing JavaScript code into reusable modules. CommonJS uses runtime require calls, while ES Modules provide standard import and export declarations with a statically described dependency structure. Their loading and interoperability rules depend on the runtime and file configuration.

Key points:

• CommonJS uses require and module.exports.

• ES Modules use import and export.

• Modules are generally cached; shared module state can affect many requests.

• Node can identify ESM through .mjs or a package's type setting; do not mix syntaxes blindly.

• Caching applies to resolved module identities; multiple copies or distinct module URLs may not share one instance.

Interview point:

• Module-level mutable state can be shared by many requests in one process, but it is not a shared store across app processes.