Appending to an already existing file nodejs?

I am aware of questions like How to append to a file in Node?

However those don't do what I need. What I have is a textfile that already contains text before nodejs is started, then I want node to append text at the end of my file.

However using the method in the question linked above overwrites the contents of my file.

I also found that I can use start:number in the options of my fs.createWriteStream so if I was to figure out where my old file ends I could use that to append, but how would I figure that out without having to read out the whole file and count the characters in it?