# Bun Write a file incrementally

> Write a file incrementally

- Skill: `jarle/bun-write-a-file-incrementally` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jarle/bun-write-a-file-incrementally`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jarle/bun-write-a-file-incrementally/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jarle (https://skillmd.com/u/jarle)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/jarle/bun-write-a-file-incrementally

---


# Write a file incrementally

Bun provides an API for incrementally writing to a file. This is useful for writing large files, or for writing to a file over a long period of time.

Call `.writer()` on a `BunFile` to retrieve a `FileSink` instance. This instance can be used to efficiently buffer data and periodically "flush" it to disk. You can write & flush many times.

```ts  theme={"theme":{"light":"github-light","dark":"dracula"}}
const file = Bun.file("/path/to/file.txt");
const writer = file.writer();

writer.write("lorem");
writer.write("ipsum");
writer.write("dolor");

writer.flush();

// continue writing & flushing
```

***

The `.write()` method can accept strings or binary data.

```ts  theme={"theme":{"light":"github-light","dark":"dracula"}}
w.write("hello");
w.write(Buffer.from("there"));
w.write(new Uint8Array([0, 255, 128]));
writer.flush();
```

***

The `FileSink` will also auto-flush when its internal buffer is full. You can configure the buffer size with the `highWaterMark` option.

```ts  theme={"theme":{"light":"github-light","dark":"dracula"}}
const file = Bun.file("/path/to/file.txt");
const writer = file.writer({ highWaterMark: 1024 * 1024 }); // 1MB
```

***

When you're done writing to the file, call `.end()` to auto-flush the buffer and close the file.

```ts  theme={"theme":{"light":"github-light","dark":"dracula"}}
writer.end();
```

***

Full documentation: [FileSink](/runtime/file-io#incremental-writing-with-filesink).