Node.js Filesystem Basics

What you will practice

• Understand filesystem access as a Node.js feature.
• Read fake file contents safely in the lesson.
• Parse file-like JSON data.

What this means

Unlike browser JavaScript, Node.js can read and write files on the machine where it runs.

That power is useful for scripts, logs, uploads, build tools, and configuration.

It is also sensitive: server code must validate paths and never let users read arbitrary files.

Mini glossary

Filesystem

The files and folders available to a program.

Path

A string that points to a file or folder location.

JSON config

A settings file written as JSON.

How it works step by step

1. Create a `data` folder and a JSON file.
2. Use `readFile` from `node:fs/promises` to read the file asynchronously.
3. Parse the text with `JSON.parse`.
4. Handle errors with `try/catch` so the script fails clearly instead of crashing mysteriously.

Where you will use this

• A build script reads a config file.
• A CLI tool loads input from a local file.
• A backend stores uploaded files after checking size and type.

Before you run the code

Modern Node.js code often imports built-in modules with the `node:` prefix, such as `node:fs/promises`.

Never build file paths directly from untrusted user input without strict validation.

Run it locally

These examples are meant for a local Node.js project. Create the files shown below, then run the terminal commands from the project folder.

mkdir lumio-fs-demo
cd lumio-fs-demo
mkdir data
touch data/settings.json read-settings.js

{
  "siteName": "Lumio Utils",
  "theme": "dark",
  "features": ["tools", "learn", "analytics"]
}

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

async function main() {
  try {
    const raw = await readFile("./data/settings.json", "utf8");
    const settings = JSON.parse(raw);

    console.log(`Site: ${settings.siteName}`);
    console.log(`Theme: ${settings.theme}`);
    console.log(`Features: ${settings.features.join(", ")}`);
  } catch (error) {
    console.error("Could not read settings:", error.message);
    process.exitCode = 1;
  }
}

main();

node read-settings.js

Common beginner mistakes

• Letting users choose arbitrary server file paths.
• Assuming a file always exists.
• Parsing JSON without handling invalid JSON errors.

Try changing this next

• Change the file path and watch the error.
• Add another setting to `data/settings.json`.
• Wrap `JSON.parse` in try/catch.