How can I select all options of multi-select select box on click?

For jQuery versions 1.6+ then

$('#select_all').click( function() {
    $('#countries option').prop('selected', true);
});

Or for older versions:

$('#select_all').click( function() {
    $('#countries option').attr('selected', 'selected');
});

LIVE DEMO

Try this:

$('#select_all').click(function() {
    $('#countries option').prop('selected', true);
});

And here's a live demo.

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