Question

I want to stop the user to check another checkbox after a certain number of checkboxes have been checked already. i.e. After 3 checkboxes are checked, the user cannot check anymore and a message says ‘You’re not allowed to choose more than 3 boxes.’

I’m almost there but the last checkbox is still being checked and I don’t want that, I want it to be unchecked with the message appearing.

How do I do that:

var productList = $('.prod-list'),
    checkBox = productList.find('input[type="checkbox"]'),
    compareList = $('.compare-list ul');

productList.delegate('input[type="checkbox"]', 'click', function () {
  var thisElem = $(this),
      thisData = thisElem.data('compare'),
      thisImg = thisElem.closest('li').find('img'),
      thisImgSrc = thisImg.attr('src'),
      thisImgAlt = thisImg.attr('alt');

  if (thisElem.is(':checked')) {
    if ($('input:checked').length < 4) {

        compareList.append('<li data-comparing="' + thisData + '"><img src="' + thisImgSrc + '" alt="'+ thisImgAlt +'" /><li>');

    } else {
        $('input:checked').eq(2).attr('checked', false);
        alert('You\'re not allowed to choose more than 3 boxes');
    }
  } else {
    var compareListItem = compareList.find('li');

    for (var i = 0, max = compareListItem.length; i < max; i++) {
        var thisCompItem = $(compareListItem[i]),
            comparingData = thisCompItem.data('comparing');

        if (thisData === comparingData) {
            thisCompItem.remove();
        }
    }

  }

});

Answer 1

I might have misunderstood the question… see my comment.

Too prevent the selection, you can call event.preventDefault() and define the handler with the event parameter.

$('input[type="checkbox"]').click(function(event) {
    if (this.checked && $('input:checked').length > 3) {
        event.preventDefault();
        alert('You\'re not allowed to choose more than 3 boxes');
    }
});

DEMO

Alternatively, set this.checked to false. This will even prevent the browser from rendering the checkmark.

DEMO