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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
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();
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
When you’re done writing to the file, call .end() to auto-flush the buffer and close the file. 
writer.end();
Full documentation: FileSink.