Global Objects and the Process

Global Objects and the Process

In the browser, JavaScript has a global object called window. In Node.js, the main global object is process. It represents the running program itself. You can ask it things like:

• process.argv — the words typed after the node command, such as a file name.
• process.env — environment variables, which are settings your computer passes to programs.

Another handy one is console, which prints messages, and __dirname, which tells you the folder your file lives in.

Reading and Writing Files

Real programs need to save and load data. Node.js includes a tool called the fs module ("fs" stands for file system) that reads and writes files.

const { readFile } = require('node:fs/promises');

async function main() {
  const text = await readFile('notes.txt', 'utf8');
  console.log(text);
}
main();

Working Asynchronously

Here is the key idea: reading a file takes time, so Node.js does not sit and wait. It starts the work, moves on, and comes back when the file is ready. This is called asynchronous code — "async" for short.

The modern way to write it is async/await. You mark a function as async, then use await to pause just that function until the work finishes, without freezing the whole program.

const { writeFile } = require('node:fs/promises');

async function main() {
  await writeFile('notes.txt', 'Hello from Node.js!');
  console.log('Saved.');
}
main();

Handling Errors in Async Code

Files can be missing, disks can fail. With async/await, you handle problems with a normal try/catch block — the same tool used for ordinary JavaScript errors.

try {
  const text = await readFile('missing.txt', 'utf8');
} catch (err) {
  console.error('Could not read file:', err.message);
}

Building Your First Web Server

A web server is a program that answers requests from browsers. Node.js has a built-in http module for this. Here is a complete, tiny server:

const http = require('node:http');

const server = http.createServer((req, res) => {
  res.end('Hello, world!');
});

server.listen(3000);
console.log('Server running at http://localhost:3000');

Run it with node server.js, then open http://localhost:3000 in your browser. You have just served a web page with no frameworks at all.

100%

Show diagram source

flowchart LR
  A[Browser] --> B[http module]
  B --> C[Your handler function]
  C --> D[Response sent back to browser]