Question

I have a table cell in the footer that allows the user to turn on row coloring:

$('#highlight').click(function() {
$(this).parents('table').RowColors();
})

// From Chapter 7 of Learning jQuery
$.fn.RowColors = function() {
$('tbody tr:odd', this).removeClass('even').addClass('odd');
$('tbody tr:even', this).removeClass('odd').addClass('even');
return this;
};

Q: How do I write a selector that says: IF there is at least 1 row with class=”even”, then remove both “even” and “odd” ELSE execute the RowColors function.

Answer 1

My advice would be to do it slightly differently. Have just one class with the default state being the other. So:

tr td { background: yellow; }
tr.odd td { background: blue; }

and then this is as simple as:

$("tr").toggleClass("odd");

or more specifically:

$("tbody > tr").removeClass("odd").filter(":nth-child(odd)").addClass("odd");

Note: avoid using :odd and :even. They usually don’t mean what you think they mean. :nth-child(odd) and :nth-child(even) tend to be what you really mean.

I would probably write something like:

$("#highlight").click(function() {
  $(this).closest("table").children("tbody").children("tr").removeClass("odd")
    .filter(":nth-child(odd)").addClass("odd");
  return false;
});

Put it into a separate function if you wish.

Edit: to check whether something is empty:

var odd = $(".odd");
if (odd.length == 0) {
  // do one thing
} else {
  // do something else
}

jQuery objects support the length property and the size() method, which do the same thing.