Asynchronous File System (fs & fs/promises)

Reading and Writing Files

The built-in fs module allows Node.js applications to interact with the file system. It provides synchronous methods, callback-based methods, and modern Promise-based APIs.

Callback vs Promises API

Avoid synchronous methods (fs.readFileSync) in production servers because they block the single main execution thread!

// Recommended: fs/promises with async/await
const fs = require("node:fs/promises");
const path = require("node:path");

async function processConfigFile() {
  const filePath = path.join(__dirname, "config.json");

  try {
    // Write JSON file
    const data = { env: "production", port: 5000 };
    await fs.writeFile(filePath, JSON.stringify(data, null, 2), "utf-8");

    // Read JSON file back
    const content = await fs.readFile(filePath, "utf-8");
    const parsed = JSON.parse(content);
    console.log("Config loaded:", parsed.port);

    // Check file existence
    await fs.access(filePath);
    console.log("File exists!");
  } catch (err) {
    console.error("File I/O Error:", err.message);
  }
}
processConfigFile();

Common File System Operations

• fs.readFile(path, encoding) : Reads entire file contents.
• fs.writeFile(path, data) : Writes data to file (overwrites existing).
• fs.appendFile(path, data) : Appends data to end of file.
• fs.mkdir(path, { recursive: true }) : Creates directory (including parent folders if needed).
• fs.unlink(path) : Deletes a file.

TL;DR

• Always prefer fs/promises with async/await over sync methods in backend applications.
• Specify encoding (e.g. 'utf-8') when reading files as strings.