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bun.sh/docs/guides/write-file/append.mdx
2025-11-05 11:14:21 -08:00

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---
title: Append content to a file
sidebarTitle: Append to file
mode: center
---
Bun implements the `node:fs` module, which includes the `fs.appendFile` and `fs.appendFileSync` functions for appending content to files.
---
You can use `fs.appendFile` to asynchronously append data to a file, creating the file if it does not yet exist. The content can be a string or a `Buffer`.
```ts
import { appendFile } from "node:fs/promises";
await appendFile("message.txt", "data to append");
```
---
To use the non-`Promise` API:
```ts
import { appendFile } from "node:fs";
appendFile("message.txt", "data to append", err => {
if (err) throw err;
console.log('The "data to append" was appended to file!');
});
```
---
To specify the encoding of the content:
```js
import { appendFile } from "node:fs";
appendFile("message.txt", "data to append", "utf8", callback);
```
---
To append the data synchronously, use `fs.appendFileSync`:
```ts
import { appendFileSync } from "node:fs";
appendFileSync("message.txt", "data to append", "utf8");
```
---
See the [Node.js documentation](https://nodejs.org/api/fs.html#fspromisesappendfilepath-data-options) for more information.