JavaScript id output only displays for a second then dissapears
<p>I am making a simple JavaScript function where you enter two numbers in a form and then the function determines if the sum of the two numbers is a prime. It all works but when the function displays a message with the document.getElementById("demo").innerHTML and </p> it is only on the web page for one second then disappears. I want to make it so the message stays until you click on the button again. I have searched this site and other site but have not been able to find anything. </p>

<p>Here is the code:</p>

<pre><code>&lt;!DOCTYPE html&gt;
&lt;html&gt;
&lt;head&gt;
&lt;link href="/prime.css" rel="stylesheet" type="text/css" /&gt;
&lt;script&gt;
function prime()
 {
  // get the values from the form
 var x= document.forms["frm1"]["x"].value; 
 var y= document.forms["frm1"]["y"].value;
  //test the values of the form
 if (x == "null"|| x=="" || isNaN(x)|| y=="null" || y==""|| isNaN(y)){
 alert("You must enter two numbers.");
 exit;
 }
  //change variables to number and add them together
  var number = (parseInt(x)+ parseInt(y));
   var a="";
 var prime = true;

  //check to make sure number is not less than one
 if (number &lt;= 1){
 alert("Sorry " +number+ " is not a prime.");
 prime = false;
 exit;
 }
   //check if number is a prime
for (var dividby = 2; dividby &lt;= number / 2; dividby++){
if(number % dividby == 0)
    prime = false;
break;

}
    //send congratulations
if(prime==true){

a="congratulations " + number + " is a prime!";
}
   //send condolences
else{
a="Sorry "+number+ " is not a prime.";
}
document.getElementById("demo").innerHTML=a;

}

&lt;/script&gt;
&lt;/head&gt;

&lt;body id="elem"&gt;
&lt;div class="div"&gt;&lt;/div&gt;
&lt;div&gt; 
    &lt;br&gt;
&lt;h1&gt;PRIME NUMBERS&lt;/h1&gt;
&lt;p&gt;Please enter two numbers to be added and see if the result is a prime.&lt;/p&gt;&lt;br&gt;
&lt;form name="frm1" &gt;
&lt;input type="text" name="x"&gt; 
+ &lt;input type="text" name="y"&gt;
&lt;button onclick="prime()"&gt;click me&lt;/button&gt;
&lt;/form&gt;
&lt;br&gt;
&lt;p id="demo"&gt;&lt;/p&gt;
&lt;/div&gt;
&lt;/body&gt;
&lt;/html&gt;
</code></pre>
