Writing files with Node.js

Writing a file

The easiest way to write to files in Node.js is to use the fs.writeFile() API.

const import fsfs = require: anyrequire('node:fs');

const const content: "Some content!"content = 'Some content!';

import fsfs.writeFile('/Users/joe/test.txt', const content: "Some content!"content, err: anyerr => {
  if (err: anyerr) {
    var console: Consoleconsole.Console.error(...data: any[]): void[MDN Reference](https://developer.mozilla.org/docs/Web/API/console/error_static)error(err: anyerr);
  } else {
    // file written successfully
  }
});

Writing a file synchronously

Alternatively, you can use the synchronous version fs.writeFileSync():

const import fsfs = require: anyrequire('node:fs');

const const content: "Some content!"content = 'Some content!';

try {
  import fsfs.writeFileSync('/Users/joe/test.txt', const content: "Some content!"content);
  // file written successfully
} catch (var err: unknownerr) {
  var console: Consoleconsole.Console.error(...data: any[]): void[MDN Reference](https://developer.mozilla.org/docs/Web/API/console/error_static)error(var err: unknownerr);
}

You can also use the promise-based fsPromises.writeFile() method offered by the fs/promises module:

const import fsfs = require: anyrequire('node:fs/promises');

async function function example(): Promise<void>example() {
  try {
    const const content: "Some content!"content = 'Some content!';
    await import fsfs.writeFile('/Users/joe/test.txt', const content: "Some content!"content);
  } catch (function (local var) err: unknownerr) {
    var console: Consoleconsole.Console.log(...data: any[]): void[MDN Reference](https://developer.mozilla.org/docs/Web/API/console/log_static)log(function (local var) err: unknownerr);
  }
}

function example(): Promise<void>example();

By default, this API will replace the contents of the file if it does already exist.

You can modify the default by specifying a flag:

fs.writeFile('/Users/joe/test.txt', content, { flag: stringflag: 'a+' }, err: anyerr => {});

The flags you'll likely use are

Flag	Description	File gets created if it doesn't exist
`r+`	This flag opens the file for reading and writing	❌
`w+`	This flag opens the file for reading and writing and it also positions the stream at the beginning of the file	✅
`a`	This flag opens the file for writing and it also positions the stream at the end of the file	✅
`a+`	This flag opens the file for reading and writing and it also positions the stream at the end of the file	✅

You can find more information about the flags in the fs documentation.

Appending content to a file

Appending to files is handy when you don't want to overwrite a file with new content, but rather add to it.

Examples

A handy method to append content to the end of a file is fs.appendFile() (and its fs.appendFileSync() counterpart):

const import fsfs = require: anyrequire('node:fs');

const const content: "Some content!"content = 'Some content!';

import fsfs.appendFile('file.log', const content: "Some content!"content, err: anyerr => {
  if (err: anyerr) {
    var console: Consoleconsole.Console.error(...data: any[]): void[MDN Reference](https://developer.mozilla.org/docs/Web/API/console/error_static)error(err: anyerr);
  } else {
    // done!
  }
});

Example with Promises

Here is a fsPromises.appendFile() example:

const import fsfs = require: anyrequire('node:fs/promises');

async function function example(): Promise<void>example() {
  try {
    const const content: "Some content!"content = 'Some content!';
    await import fsfs.appendFile('/Users/joe/test.txt', const content: "Some content!"content);
  } catch (function (local var) err: unknownerr) {
    var console: Consoleconsole.Console.log(...data: any[]): void[MDN Reference](https://developer.mozilla.org/docs/Web/API/console/log_static)log(function (local var) err: unknownerr);
  }
}

function example(): Promise<void>example();

Node.jsを使用したファイルの読み込み Node.jsでのフォルダーの操作