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window.onresize = function(event) {
    ...
};
//or
<script> 
      
        // Defining event listener function 
        function displayWindowSize() { 
              
            // Get width and height of the window 
            // excluding scrollbars 
            var w = document.documentElement.clientWidth; 
            var h = document.documentElement.clientHeight; 
  
            // Display result inside a div element 
            document.getElementById("result").innerHTML 
                  = "Width: " + w + ", " + "Height: " + h; 
        } 
  
        // Attaching the event listener function 
        // to window's resize event 
        window.addEventListener("resize", displayWindowSize); 
  
        // Calling the function for the first time 
        displayWindowSize(); 
    </script>