Question

The following function needs at least 3 seconds to run (on 500 table rows). Is it possible to make this function faster?

function prepareTable() {
    var groupIndex = 0;
    $("#row tbody tr").each(function(index) {
    // each row gets a unique id
    // remove default css styles for table rows
    // read out hidden value, that stores if row is a group
    var group = $(this).attr('id', 'node-'+index).removeClass("odd event").find('td :hidden').attr('value');
    // if it is a group, add special styles to row and remember row index
    if (group == 'true') {
        groupIndex = index;
        $(this).addClass('odd').find("td:first")
            .mouseenter(function() {
                $(this).parent().addClass("swGroupLink");
            })
            .mouseleave(function() {
                $(this).parent().removeClass("swGroupLink");
        });
    } else {
        // make all following rows to children of the previous group found
        $(this).addClass('even child-of-node-' + groupIndex);
    }   
    });
}

Answer 1

I suggest two improvements:

• Cache DOM References
• Work at your table offline

Example

function prepareTable() {
   var groupIndex = 0;
   var $mytable = $('#row'),
       $parent  = $mytable.parent();

   $mytable = $mytable.detach();

   $mytable.find('tr').each(function(index) {
      var $this = $(this);
      var group = $this.attr('id', 'node-'+index).removeClass("odd event").find('td :hidden').attr('value');
// if it is a group, add special styles to row and remember row index
   if (group == 'true') {
       groupIndex = index;
       $this.addClass('odd').find("td:first")
           .mouseenter(function() {
               $this.parent().addClass("swGroupLink");
           })
           .mouseleave(function() {
               $this.parent().removeClass("swGroupLink");
       });
   } else {
       // make all following rows to children of the previous group found
       $this.addClass('even child-of-node-' + groupIndex);
   }   
   });

   $parent.append($mytable);
}

I added a variable $this which caches $(this) in your .each() loop. I also added $mytable and $parent. $mytable stores the #row element and $parent stores the parent-node from #row. That is because I remove the whole element from the DOM, do the work and re-attach it to the parent.

Test environment: http://www.jsfiddle.net/2C6fB/4/

If that is still too slow, you have other options here. First, look if you can split the loop into smaller pieces. You can optimize that like a lot by using asychronous callbacks, for instance, setTimeout. That can be a tricky business and I would need to know your code in more detail, but in general you might just want to wrap your whole loop into single setTimeout() functions. Example -> http://www.jsfiddle.net/2C6fB/5/

This ensures that the browser won’t “hang” while operating. But of course this took a little bit longer to complete the whole task.