$('#checkAll').click(function() {
if(this.checked) {
$('input:checkbox').attr('checked', true);
}
else {
$('input:checkbox').removeAttr('checked');
}
});
$('input:checkbox:not(#checkAll)').click(function() {
if(!this.checked) {
$('#checkAll').removeAttr('checked');
}
else {
var numChecked = $('input:checkbox:checked:not(#checkAll)').length;
var numTotal = $('input:checkbox:not(#checkAll)').length;
if(numTotal == numChecked) {
$('#checkAll').attr('checked', true);
}
}
});
Demo: http://jsfiddle.net/ThiefMaster/HuM4Q/
As pointed out in the question's comment, a regular checkbox is not perfect for this. My implementation disables the "check all" box as soon as one checkbox is unchecked. So, to uncheck all still-checked checkboxes you'll have to click twice (first to re-check the unchecked ones and then to uncheck all other ones).
However, with a tri-state checkbox this might still be necessary as the state order might be unchecked->indefinite->checked->unchecked, so you'd need two clicks to come from indefinite to unchecked.
Since you probably don't want to check ALL checkboxes on your page with "Check All", replace input:checkbox
with e.g. .autoCheckBox
or input.autoCheckBox:checkbox
and give those checkboxes class="autoCheckBox"
.
If you want all checkboxes inside a certain form, simple use #idOfYourForm input:checkbox
or form[name=nameOfYourForm] input:checkbox