Node.js is a JavaScript runtime built on Chrome's V8 engine. It allows you to run JavaScript on servers, in scripts, and anywhere outside the browser.

Node.js uses non-blocking I/O operations. While waiting for I/O (file reads, network requests), Node continues processing other tasks. The event loop manages callbacks when operations complete.

npm init creates a package.json file that stores your project's metadata, dependencies, and npm scripts.

Local files must use relative paths starting with ./ or ../ to distinguish them from npm packages. Without the ./, Node looks for an npm package named 'utils'.

The async keyword marks a function as asynchronous. Async functions always return a Promise and allow the use of 'await' inside them.

readFileSync blocks the event loop until the file is completely read, then returns the contents directly. readFile is non-blocking and uses a callback or Promise.

URL parameters defined with :name are accessed via req.params. Query strings (?id=1) use req.query, and POST body data uses req.body.

Middleware functions have access to the request and response objects. They can modify them, end the request cycle, or call next() to pass control to the next middleware.

node_modules can be very large (100MB+) and is regenerated by running 'npm install' from package.json. Everyone should add node_modules/ to their .gitignore.

res.json() automatically sets the Content-Type header to application/json and stringifies the object. It's the cleanest way to return JSON in Express.

Promise.all() takes an array of promises and runs them concurrently. It returns a single Promise that resolves when all input promises resolve, with an array of their results.

The event loop continuously checks for completed I/O operations, timers, and other async events, executing their callbacks. It keeps Node.js running until there's nothing left to do.