I want to convert jquery code to vanilla code [duplicate]

my english is not very good

jquery 1.5.2 is required for this code. I wonder if you could change it to vanilla, which doesn’t require a library? I don’t want to download 30 kilobytes of file for so little code. thanks in advance

$(function() {
   $('a').click(function() {
       $(this).addClass('active');
   });
});