window.innerHeight

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
    <script src="script.js"></script>
  </head>

  <body onload="getHeight()">
    <h3>
      the current window height is : <span id="height"></span>px
    </h3>
    
    you can change the browser's height and click <button onclick="getHeight()">refresh</button> to get the new value
    
    <script>
      function getHeight(){
        var height = window.innerHeight;
        document.getElementById("height").innerHTML = window.innerHeight;
      }
    </script>
    
  </body>

</html>

// Code goes here

/* Styles go here */