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2026-08-27 21:09:14 +00:00

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---
title: Write a file incrementally
sidebarTitle: Incremental write
mode: center
---
Bun provides an API for incrementally writing to a file. Use it for large files, or when writing to a file over a long period of time.
Call `.writer()` on a `BunFile` to retrieve a `FileSink` instance. The `FileSink` buffers data. Call `.flush()` to write the buffer to disk. You can write & flush many times.
```ts
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 accepts strings or binary data.
```ts
writer.write("hello");
writer.write(Buffer.from("there"));
writer.write(new Uint8Array([0, 255, 128]));
writer.flush();
```
---
The `FileSink` also auto-flushes when its internal buffer is full.
---
When you're done writing, call `.end()` to flush the buffer and close the file.
```ts
writer.end();
```
---
Full documentation: [FileSink](/runtime/file-io#incremental-writing-with-filesink).