14 — Modules

One line: Node runs two module systems side by side, and a file's extension or the nearest package.json settles which one it gets before a single line of it runs.

The chapter starts with the two systems and how Node picks one for each file. Then the ES module syntax, in the order a reader meets it: import and export, the live bindings they create, import() and top-level await, and import.meta. Packages come next, whose exports field decides which files outsiders may load, then require() of an ES module, the bridge between the systems. Cycles come last, because they make the loading order visible.

The pages

Page	Level	In one line	Status
ES modules and CommonJS	101	Node settles a file's module system before running it — .mjs is ESM, .cjs is CommonJS, .js follows the nearest package.json "type" or else its own syntax — and then one line differs: typeof require is "function" in CommonJS, "undefined" in ESM.	stub
import and export	101	Imports are linked before any line runs, so importing a name the module does not export fails with a SyntaxError before the file's first console.log, and a default export is only an export named default: import { default as d } reads it.	stub
Live bindings	201	An imported name is a read-only view of the exporter's variable: after the exporter runs count += 1 the importer reads 1, assigning to the import throws a TypeError, and a CommonJS destructured copy still reads 0.	stub
Dynamic import() and top-level await	201	import() is an expression that returns a promise, so it works even in CommonJS, while top-level await is ESM-only and holds up every importer: a module that awaits a timer finishes before its importer's first line runs.	stub
import.meta	201	import.meta.url is a file: URL, not a path — a folder named meta dir comes back as meta%20dir — and Node adds filename, dirname, resolve() and main to the same object, which a .cjs file cannot even mention.	stub
package.json	201	Once a package has an "exports" field, any file it does not list is off limits — require("greet/internal.cjs") throws ERR_PACKAGE_PATH_NOT_EXPORTED although the file is on disk — and its import and require conditions can give the two systems different files.	stub
require and ES modules	301	On Node 24 and 25, require() of an ES module returns its namespace object — unless the module uses top-level await, which throws ERR_REQUIRE_ASYNC_MODULE — and import of a CommonJS file gives module.exports as the default export.	stub
Module cycles	301	In a cycle one module runs before the other has finished: an ES module reading the other's const at top level throws ReferenceError (the binding is still in its temporal dead zone), while CommonJS hands over a half-filled exports and reads undefined.	stub

Boundaries

Turning import.meta.url into a file path belongs to the Node runtime chapter, and how tsc resolves the same specifiers belongs to the module-resolution chapter.