Add append content to a file guide (#6581)

* Add append content guide

Resolve #6559

* Update guide

---------

Co-authored-by: Colin McDonnell <colinmcd94@gmail.com>

1 files changed, 52 insertions, 0 deletions

diff --git a/docs/guides/write-file/append.md b/docs/guides/write-file/append.md
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+---
+name: Append content to a file
+---
+
+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.