3. CommonJS modules

Node's original module system is CommonJS: each file is its own module, and you export things with `module.exports` (or the shorthand `exports.foo = ...`) and pull them in with `require`.

js
// math.js
function add(a, b) { return a + b; }
module.exports = { add };

// index.js
const { add } = require("./math");
console.log(add(2, 3));

`require` is synchronous and cached: the first call executes the module and caches its `exports`; every later `require` of the same path returns the cached object instantly, so top-level module code runs exactly once no matter how many files import it.

Node resolves bare specifiers like `require("lodash")` by walking up `node_modules` directories, and relative specifiers like `require("./math")` relative to the current file.

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