How to write a string to the middle of a file in NodeJS?

Question

JavaScript

Is it possible to write a line in the middle of a file without overwriting the entire file?
For example, there is a test.txt file with the content:
1234567890abcdfghijklmnoprstxyz
You need to write the string "++" instead of "ab", that is, what would happen:
1234567890++cdfghijklmnoprstxyz
I try this:

var fs = require('fs');
var path = require('path');

var fd = fs.openSync(path.join(process.cwd(), 'test.txt'), 'a+');

var buf=Buffer.from("++","utf8");

fs.writeSync(fd, buf, 0,2, 10);

console.log('closing file now')
fs.closeSync(fd);

But "++" is written to the end of the file, changing the fs.writeSync parameters does not help, it still writes a line to the end of the file. Although according to the documentation https://nodejs.org/api/fs.html#fs_fs_writesync_fd_... , there is a position parameter.
And in fs.readSync(fd, buf, 0,2,10); position works, and reads the string from the desired position.