How to detect file ends in newline?

Here is a useful bash function:

function file_ends_with_newline() {
    [[ $(tail -c1 "$1" | wc -l) -gt 0 ]]
}

You can use it like:

if ! file_ends_with_newline myfile.txt
then
    echo "" >> myfile.txt
fi
# continue with other stuff that assumes myfile.txt ends with a newline