What are the differences between ES modules and CommonJS?

CommonJS (CJS) is Node's original module system. ES modules (ESM) are the official JavaScript standard. The main difference: CommonJS loads at runtime and can be anywhere, while ES module imports are static and can be analyzed before the code runs.

CommonJS:

• It uses require() and module.exports.
• Loading is synchronous and happens at runtime. So require can appear anywhere, even inside an if.
• You get a copy of the exported value at the moment of the call. For objects, the copy is a reference to the same object.

ES modules:

• They use import and export.
• Static import declarations must be at the top level. So tools can read the dependency graph before running the code. This enables tree shaking (removing unused code).
• Loading is asynchronous in browsers.
• Imports are live read-only bindings. "Live" means you always see the exporter's current value. "Read-only" means the importer cannot assign to it.
• Modules are always strict, top-level this is undefined, and top-level await is allowed.
• import() loads a module dynamically and returns a promise.

In Node: a file is ESM if it ends in .mjs, or if package.json has "type": "module". ESM can import CommonJS. Recent Node versions can also require() ES modules that do not use top-level await.