Not to put words in his mouth, but the point I think japhy (right?) was trying to make was that, if the reason you're writing to the beginning of the file is that you think that there's where you should store the most "recent" information, then there's really no point.

Just keep your file in chronological order (oldest at beginning, newest at end) and, when you're reading the file into memory, use unshift rather than push so that the newest data ends up at the beginning of the array.

Which definitely makes sense, and makes the problem quite a lot less complicated, if that's the reason the op wanted to write to the beginning of the file.