File System

Three flavours of the fs module

Node ships the same filesystem operations in three distinct APIs:

Style	Import	When to use
Callback (async)	`import fs from 'node:fs'`	Legacy code, Node streams
Synchronous	`import fs from 'node:fs'`	CLI scripts, startup-time config
Promise (async)	`import fs from 'node:fs/promises'`	All new server code

Reading a file

Callback style

import fs from 'node:fs';

fs.readFile('./hello.txt', 'utf8', function(err, data) {
  if (err) throw err;
  console.log(data);
});

Promise style (recommended)

Node.js

import { writeFile, readFile, readdir, rm } from 'node:fs/promises';

async function demo() {
  // Write a file
  await writeFile('/tmp/demo.txt', 'Hello from Node.js!', 'utf8');
  console.log('Written: /tmp/demo.txt');

  // Read it back
  var content = await readFile('/tmp/demo.txt', 'utf8');
  console.log('Read back:', content);

  // List /tmp directory
  var entries = await readdir('/tmp');
  console.log('Files in /tmp (first 5):', entries.slice(0, 5));

  // Clean up
  await rm('/tmp/demo.txt');
  console.log('Deleted /tmp/demo.txt');
}

demo().catch(console.error);

Needs the Node.js runtime — open in StackBlitz to run.

Writing a file

writeFile creates the file if it does not exist and replaces the contents if it does. Use appendFile to add to an existing file.

Node.js

import { writeFile, appendFile, readFile, rm } from 'node:fs/promises';

async function demo() {
  var path = '/tmp/log.txt';

  await writeFile(path, 'Line 1\n', 'utf8');
  await appendFile(path, 'Line 2\n', 'utf8');
  await appendFile(path, 'Line 3\n', 'utf8');

  var result = await readFile(path, 'utf8');
  console.log('File contents:');
  console.log(result);

  await rm(path);
}

demo().catch(console.error);

Needs the Node.js runtime — open in StackBlitz to run.

Checking existence and stats

Node.js

import { stat, writeFile, rm } from 'node:fs/promises';

async function demo() {
  var path = '/tmp/stat-test.txt';
  await writeFile(path, 'stat demo', 'utf8');

  var info = await stat(path);
  console.log('size (bytes):', info.size);
  console.log('isFile:', info.isFile());
  console.log('isDirectory:', info.isDirectory());
  console.log('modified:', info.mtime.toISOString());

  await rm(path);
}

demo().catch(console.error);

Needs the Node.js runtime — open in StackBlitz to run.

Synchronous reads — startup only

import { readFileSync } from 'node:fs';

// Acceptable at startup — process has not yet started accepting requests
const config = JSON.parse(readFileSync('./config.json', 'utf8'));

Never call readFileSync inside a request handler, route callback, or any function invoked after the server starts.