Execute javascript code when R shiny output have been rendered

If you have reactive observer like this:

output$resultsTable <- renderTable({
   tableData
});

and you want to modify the table from javascript you can’t put shinyjs::runjs after table is rendered because the table data need to be the last expression of the reactive renderTable observer. The solution is to use new feature of JavaScript, implemented in browsers, which is mutation observer. The browser support is very good, you can check it on caniuse. The solution that will work, look like this (using jQuery plugin):

$.fn.onRender = function(callback, options) {
  if (arguments[0] == 'cancel') {
    return this.each(function() {
      var observer = $(this).data('observer');
      if (observer) {
        observer.disconnect();
      }
    });
  } else {
    var settings = $.extend({
      oneTime: true,
      observer: {childList: true, subtree: true}
    }, options);
    return this.onRender('cancel').each(function() {
      var node = $(this);
      var observer = new MutationObserver(function(mutations) {
        callback.apply(node[0]);
        if (settings.oneTime) {
          observer.disconnect();
        }
      });
      observer.observe(node[0], settings.observer);
    });
  }
};

You can use it like this:

function changeTable(selector) {
   $(selector).onRender(function() {
      $(this).find('thead th').css('color', 'red');
   });
}

if you want to execute code once and use it outside of shiny observer you can use option oneTime: false like this:

function updateTableWhenRender(selector) {
   $(selector).onRender(function() {
      $(this).find('thead th').css('color', 'red');
   }, {oneTime: false});
}

then the function can be put outside of observer like this:

output$resultsTable <- renderTable({
   tableData
});
shinyjs::runjs("updateTableWhenRender('#resultsTable')")

To cancel the mutation observer, that’s create with the plugin, you can execute this line of code:

$(selector).onRender('cancel');
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