check / uncheck checkbox using jquery?

For jQuery 1.6+ :

.attr() is deprecated for properties; use the new .prop() function instead as:

$('#myCheckbox').prop('checked', true); // Checks it
$('#myCheckbox').prop('checked', false); // Unchecks it

For jQuery < 1.6:

To check/uncheck a checkbox, use the attribute checked and alter that. With jQuery you can do:

$('#myCheckbox').attr('checked', true); // Checks it
$('#myCheckbox').attr('checked', false); // Unchecks it

Cause you know, in HTML, it would look something like:

<input type="checkbox" id="myCheckbox" checked="checked" /> <!-- Checked -->
<input type="checkbox" id="myCheckbox" /> <!-- Unchecked -->

However, you cannot trust the .attr() method to get the value of the checkbox (if you need to). You will have to rely in the .prop() method.