Continous running Java Script?

programming web dev
1

For some reason this function runs continuosly? can someone see something wrong with this script, and why it runs continuosly?

<html xmlns = "http://www.w3.org/1999/xhtml">
  <head>
     <title>Temperature</title>
        <script type = "text/javascript">
 
function popup(){ 
 
 var fahrTemp = window.prompt( "Please enter the Current Temperature in fahrenheit.", "0" ); 
 
 fahrTemp = parseInt(( fahrTemp - 32)*(.55)); 
 
 document.write( "<h3>The current Temperature is " + fahrTemp + " Degrees Celsius</h3>" ); 
  
  }
 </script>
   </head>
   <body>
 
 <h3> Click the button below to see the Current Fahrenheit Temperature converted into Celsius.</h3>
 
 <input type="button" onclick="popup()" value="Temperature Conversion" >
 
   </body>
</html>