Files, stdin, and process

The fs module

Browsers cannot touch your files. Node can, through the built-in fs (file system) module.

const fs = require("node:fs");

fs.writeFileSync("notes.txt", "hello");        // create or overwrite
const text = fs.readFileSync("notes.txt", "utf8"); // read as text

The Sync suffix means blocking: the whole thread stops until the disk answers. Fine for a startup script, bad inside a busy server (lesson 2-2 told you why). The non-blocking versions take a callback:

fs.readFile("notes.txt", "utf8", (err, data) => {
  if (err) return console.error("could not read");
  console.log(data);
});

Note the error-first callback: Node convention puts a possible error as the first argument, the result second. Always check err before touching data.

The process object

Every Node program gets a global process object describing the running program:

• process.argv: the command-line arguments (node app.js --port 4000).
• process.env: environment variables, key-value settings from the outside world. This is how servers receive secrets and config (unit 8 builds on it).
• process.stdin: the standard input stream, text piped into your program.

process.stdin is a stream that fires callbacks as data arrives, the same event style servers use for incoming requests:

let input = "";
process.stdin.on("data", (chunk) => { input += chunk; });
process.stdin.on("end", () => {
  // all input has arrived, use it here
});

.on(name, callback) means "when the name event happens, run this". You will meet .on again the moment we open a real HTTP server in unit 3.