Write a file incrementally

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. It buffers data; call .flush() to write the buffer to disk. You can write & flush many times.

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.

w.write("hello");
w.write(Buffer.from("there"));
w.write(new Uint8Array([0, 255, 128]));
writer.flush();

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

const file = Bun.file("/path/to/file.txt");
const writer = file.writer({ highWaterMark: 1024 * 1024 }); // 1MB

When you’re done writing, call .end() to flush the buffer and close the file.

writer.end();

Full documentation: FileSink.