How do you write data to a file?

Node.jsJunior~2 min read

1. Asynchronous writing (via a callback)

The classic way, doesn't block the thread, good for servers.

javascript

import fs from 'fs';

fs.writeFile('./output.txt', 'Hello, Node.js!', (err) => {
  if (err) {
    console.error('Write error:', err);
    return;
  }
  console.log('File written successfully!');
});

If the file doesn't exist, Node creates it.
If it already exists, its content gets overwritten.
A third argument (options) can be passed, for example { flag: 'a' } to append (see below).

2. Asynchronous writing with promises (`fs/promises`)

The modern, readable, and recommended option. Works with async/await, no callbacks.

javascript

import { writeFile } from 'fs/promises';

try {
  await writeFile('./output.txt', 'Hello from fs/promises!');
  console.log('File created!');
} catch (err) {
  console.error('Error:', err);
}

Options can be added:

javascript

await writeFile('./log.txt', 'First line\n', { flag: 'a' }); // "append"

3. Appending data to the end of a file

To add data rather than overwrite it:

javascript

import fs from 'fs';

fs.appendFile('./log.txt', 'A new entry\n', (err) => {
  if (err) throw err;
  console.log('Entry appended!');
});

or with await:

javascript

import { appendFile } from 'fs/promises';
await appendFile('./log.txt', 'Added another line\n');

4. Synchronous writing (blocking)

Rarely used (for example, when loading configuration). Blocks the event loop, not recommended for server applications.

javascript

import fs from 'fs';

try {
  fs.writeFileSync('./output.txt', 'Synchronous write');
  console.log('File written!');
} catch (err) {
  console.error(err);
}

Program execution is halted until the file is written.

5. Streaming writes (for large files)

If a large amount of data needs writing (for example, a log or a big JSON file), streams are the better choice.

javascript

import fs from 'fs';

const stream = fs.createWriteStream('./bigdata.txt');

stream.write('First line\n');
stream.write('Second line\n');
stream.end('File complete!\n');

stream.on('finish', () => console.log('Write finished!'));
stream.on('error', (err) => console.error('Error:', err));

Advantages of streams:

Writing happens in chunks, no need to hold everything in memory;
Useful for streaming HTTP responses or logging.

6. Write options (`options`)

writeFile, appendFile, and createWriteStream accept a settings object:

javascript

{
  encoding: 'utf8',  // encoding
  mode: 0o666,       // file permissions
  flag: 'w'          // mode: 'w' = write, 'a' = append, 'wx' = write only if the file doesn't exist
}

Example:

javascript

await writeFile('./data.txt', 'Test', { flag: 'a', encoding: 'utf8' });

7. Example: writing an object to a JSON file

javascript

import { writeFile } from 'fs/promises';

const user = { name: 'Tim', age: 28 };

await writeFile('./user.json', JSON.stringify(user, null, 2));
console.log('JSON saved!');

JSON.stringify(..., null, 2) produces a nicely indented format.
To update it, read the file → parse → update → writeFile again.

8. Checking existence before writing

javascript

import fs from 'fs';

if (fs.existsSync('./data.txt')) {
  console.log('The file already exists!');
} else {
  fs.writeFileSync('./data.txt', 'A new file was created');
}

Or asynchronously:

javascript

import { access, writeFile } from 'fs/promises';
import { constants } from 'fs';

try {
  await access('./data.txt', constants.F_OK);
  console.log('The file already exists');
} catch {
  await writeFile('./data.txt', 'Created automatically');
}

9. A "logger" example (a real use case)

javascript

import { appendFile } from 'fs/promises';

async function log(message) {
  const timestamp = new Date().toISOString();
  await appendFile('./server.log', `[${timestamp}] ${message}\n`);
}

await log('Server started');
await log('A user logged in');

A great fit for keeping logs without any libraries.

10. A summary table

Method	Asynchronous	Overwrites	Appends	Streaming	Recommended
`fs.writeFile`	Yes	Yes	No	No	Yes
`fs/promises.writeFile`	Yes (`await`)	Yes	No	No	Yes, the best option
`fs.appendFile`	Yes	No	Yes	No	Yes
`fs.createWriteStream`	Yes	Configurable	Yes	Yes	Yes (large data)
`fs.writeFileSync`	No	Yes	No	No	CLI only

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