What is the difference between CommonJS and ES modules in Node.js?

They are two module systems with different loading semantics, and mixing them is where the pain lives.

• Syntax. CommonJS uses require() and module.exports; ES modules use import and export.
• Timing. require is synchronous and can be called anywhere, including conditionally inside a function. import is static and hoisted — the module graph is resolved before any code runs, which is what makes tree-shaking possible. For a conditional load you need dynamic import(), which returns a promise.
• Bindings. CommonJS gives you a copy of the exported value at the time of the require. ES modules give you a live binding, so if the exporting module reassigns the variable later, importers see the new value.
• Enabling them. Either "type": "module" in package.json, or the .mjs extension. Under ESM there is no __dirname, __filename, or require; the equivalents come from import.meta.url.

Note: An ES module can import a CommonJS module, but CommonJS cannot require an ES module — because require is synchronous and ESM resolution is asynchronous. That asymmetry is the single most common migration problem.