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.

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

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

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

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The FileSink will also auto-flush 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

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

writer.end();

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Full documentation: FileSink.