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// global vars
var winWidth = $(window).width();
var winHeight = $(window).height();

// set initial div height / width
$('div').css({
  'width': winWidth,
  'height': winHeight,
});

// make sure div stays full width/height on resize
$(window).resize(function(){
  $('div').css({
    'width': winWidth,
    'height': winHeight,
  });
});