writeStream write prepends unknown characters to markdown file

Question

I'm trying to edit markdown files through a web interface. I can already read and parse the files in my editor successfully.

Now I'm trying to update the original file as the user is typing. I'm using createWriteStream with r+ flag. The user inputs are successfully written to the destination file, but some strange characters keeps being prepended to the file.

I assume it's some encoding issue, but can't figure out what to do.

Here's my code:

    let writer = null;
    const setNewWriteStream = path => {
      if (writer) {
        writer.end();
        writer = null;
      }
      writer = fs.createWriteStream(path, { flags: 'r+' });
    };
    const writeNewData = data => {
      fs.truncate(writer.path, 0, err => {
        if (err) {
          console.log(err);
          return;
        }
        writer.write(data);
      });
    };

So basically, everytime the user opens a file, I reset the writer and create a new one. When the user types, the writeNewData function is called, passing the whole document's text as argument. So I truncate the content and write the new one.

This is the content of my file when opened:

# Overview

This is after being updated:

I've tried explicitly setting the encoding to 'utf8' but with no success. I've searched around the doc for options and their descriptions but I must say I got lost in the process.

I also tried the strip-bom library thinking it might be BOM character (still not sure abot that) but with no success.

writer.write(stripBom(data)) // same result

EDIT:

It looks like this issue is only happening when using the truncate method. I tried using truncateSync but the same issue happens. If I don't use truncate at all, there's no issue except the whole content of the file is constantly appended to the previous one. Resulting in something like this:

# Overview# Overview t# Overview te# Overview tes# Overview test