shiny/srcjs/output_binding_downloadlink.js

  var downloadLinkOutputBinding = new OutputBinding();
  $.extend(downloadLinkOutputBinding, {
    find: function(scope) {
      return $(scope).find('a.shiny-download-link');
    },
    renderValue: function(el, data) {
      $(el).attr('href', data);
    }
  });
  outputBindings.register(downloadLinkOutputBinding, 'shiny.downloadLink');