Talking to the world
A useful program reads input (from the keyboard, a file, a network call, or another program) and produces output (to the screen, a file, or another network call). JavaScript provides simple helpers for the basic cases and powerful libraries for the rest.
```javascript
// Browser
const name = prompt("Your name?");
alert("Hello, " + name);// Node.js import fs from "node:fs"; const text = fs.readFileSync("notes.txt", "utf8"); ```
Buffering and performance
Reading or writing one byte at a time is usually slow. JavaScript buffers I/O behind the scenes so that small reads and writes are batched into larger ones. When you need maximum performance — large log files, streaming uploads — drop down to the buffered or streaming APIs explicitly.
Closing resources
Files, sockets and database connections must be closed when you are done with them. Forgetting leaks operating-system resources and eventually crashes long-running programs. Use the language's built-in with / using / defer construct (it has a different name in each language) to guarantee cleanup happens.