lines = []
preselected_lines = "som gaksldj jgl jsal sjaldk"

contents = File.readlines 'app.js'
inserted? = false
contents.each_with_index do |line, i|
    if i < 10 || i > 20 and !inserted?
        lines.append(preselected_lines.split("\n"))
        inserted = true
    else
        lines.append(line)
    end
end