How to hide NaN (JavaScript)

I'm learning loops in JavaScript and

I'd like to get how to hide all text strings from output.

When alert of skipping was inside @for operator there was no NaN values in output, but it was shown 2 times undesirable alerts, that's why I moved alert block out of @for loop, and now I got another issue - shows all strings... all values (6) from array..

Here my code

<script type="text/javascript">
//Made function calc degCent
function convertToCentigrade(degFahren)
{
  var degCent;
  degCent = 5/9 * (degFahren - 32);

   return degCent

}


// Created array with data
var degFahren = new Array(212, 32, -459.15, "Huy", 34, 50, "naxxx");

//Create var array degCent
var degCent = new Array ();
//Create Counter
var loopCounter;
//Create loop 

if (isNaN(degFahren))
        { 
    alert ("You have text parameters that will be skipped ");

       } 

for (loopCounter = 0; loopCounter <=6; loopCounter++)
  {   

   // Calc degCent via function 

   degCent[loopCounter] = convertToCentigrade(degFahren[loopCounter]);


   document.write ("Value " + loopCounter + " was " + degFahren[loopCounter] + " degrees Fahrenheit");
   document.write (" which is " + degCent[loopCounter] +  " degrees centigrade<br />");

  }

</script>