Node.js: closing a file after writing

Question

I'm currently getting a writable stream to a file using writer = fs.createWriteStream(url), doing a number of writes using writer.write(), and at the end I do writer.end(). I notice that when I do writer.end(), the file size is still zero, and remains at zero until the program terminates, at which point it reaches its correct size and the contents are visible and correct.

So it seems that writer.end() isn't closing the file.

The spec says "If autoClose is set to true (default behavior) on 'error' or 'finish' the file descriptor will be closed automatically."

I'm not sure what 'error' or 'finish' refer to here. Events presumably: but how do they relate to my call on writer.end()? Is there something I have to do to cause these events?

I would try getting a file descriptor directly and calling fd.close() explicitly to close it after writing, but I don't see a method to get a writeable stream given a file descriptor.

Any advice please?

Answer 1

When you call .write Node does not write immediately to the file, but it buffers all chunks until highWaterMark bytes are reached. At that point it will try to flush the contents to disk.

Reason why it's important to check .write return value, if false is returned it means that you need to wait until drain event is emitted, if you don't do this, you can exhaust the memory of the application, see: why does attempting to write a large file cause js heap to run out of memory

The same happens for .end. It won't close the file immediately, first it will flush the buffer and after everything has been written into the file, it will close the fd.

So once you call .end you'll have to wait until finish event has been emitted.

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

const { once } = require('events');
const fs = require('fs');

const writer = fs.createWriteStream('/tmp/some-file');
// using top-level await, wrap in IIFE if you're running an older version
for(let i = 0; i < 10; i++) {
   if(!writer.write('a'))
     await once(writer, 'drain');
}
writer.end();
await once(writer, 'finish');
consle.log('File is closed and all data has been flushed');