What are Node.js core modules?
Built-in modules: http/https (web server), fs (file system), path (path utilities), os (OS info), crypto (cryptography), events (event emitter), stream (streams), util (utilities), child_process (spawn processes), url (URL parsing). They don't need installation require('fs') works directly.
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More FAQs in Node.js Modules and require Interview Questions CommonJS vs ES Modules
require (CommonJS) is synchronous and traditionally used in Node.js. import (ES Modules) is asynchronous, is the JavaScript standard, and is supported in Node.js 13+ with type: 'module' in package.json. require uses module.exports, import uses export/export default.
Five steps: resolution (find the file), loading (read content), wrapping (wrap in a function with exports, require, module, __filename, __dirname), evaluation (execute the function), and caching (store for future requires). Cached modules return the same instance on subsequent requires.
When module A requires B and B requires A, Node.js returns a partial (incomplete) module.exports object. When B requires A, it gets the current state of A's exports, which may be incomplete. Best practice: avoid circular dependencies by refactoring shared logic into a third module.
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