NodeJS - How to wait until fs.WriteStream is closed

Question

I have some code that I need to wrap with mine, the original code is creating a WriteStream this way -

function createLog() {
    this.stream = this.fs.createWriteStream(this.logFilePath, {flags: 'a'});
    this.stream.write('log content');
    this.stream.close();
}

function testLog(){ 
    this.createLog();
    // Here I want to wait until the file is closed
    // something like this.stream.waitToBeClosed();
    let text = fs.readFileSync(this.logFilePath, 'utf-8');
    return text.match(
        `log content`
    );
}

then, in the end of the function the stream is closed using -

this.stream.close()

I need to use that function, and then examine the log file and verify it contain all the required data, but unfortunately sometimes the last lines are missing. I believe it's because the stream is still not closed when I open the file for reading.

I know that I can use stream.on('finish'... to execute code when the stream is closed, but I am not supposed to touch the createLog code... (As I am writing a test for it) and before the execution of the createLog function I have no access to the stream object.

Is there a way that I can wait until the stream is closed from outside the original function?

Please show us the whole function that contains this code. It's unclear from your description why you can just use the `finish` or `close` events. Perhaps we need to see more context or know more about what you can and can't modify (and why). — jfriend00, Mar 20 '23 at 23:42
Are you sure it is a close issue? Did you try a fsync? https://nodejs.org/api/fs.html#filehandledatasync — Asad Awadia, Mar 21 '23 at 02:55
@jfriend00 added code snippets to the post, is it more clear now ? — Meir Tolpin, Mar 21 '23 at 07:47

jfriend00 · Answer 1 · 2023-03-21T15:35:37.690

If you really can't modify createLog() to provide the caller a means to know when it's done (which is really the right way to do things), then because the stream is in the instance data, you could modify testLog() like this:

async function testLog() {
    this.createLog();
    // wait for the stream to complete its work
    await new Promise((resolve, reject) => {
        this.stream.on('close', resolve).on('error', reject);
    });
    let text = fs.readFileSync(this.logFilePath, 'utf-8');
    return text.match('log content`);
}

Note, that testLog() is now async and thus will return a promise that resolves with the text.match() results.

NodeJS - How to wait until fs.WriteStream is closed

1 Answers1