What is the require() function in Node.js?

require() is Node.js's built-in function for loading modules in the CommonJS module system. It takes a module identifier and returns the exported value. Module identifiers can be: (1) Built-in modules: require("fs"), require("http") — no path needed; (2) npm packages: require("express") — Node.js looks in node_modules/; (3) Local files: require("./utils") — relative path with or without .js extension. require() is synchronous — it loads the module, executes it, and returns module.exports before continuing. Node.js caches modules after the first load — subsequent require() calls for the same module return the cached export without re-executing the module file. This makes circular dependencies possible (though tricky) and module-level state work as singletons.