document.write in jscript dosent work [on hold]

I am new in jscript and i wrote the code below but document.write dosent run. could you help me please?

<script>
var numBoul=ParseInt(prompt('Entrez le numero de Boule'));
var lettre='A';
 if (numBoul>=1 && numBoul<=15)
   lettre='B';
 if (numBoul>=16 && numBoul<=30)
   lettre='I';
 if (numBoul>=31 && numBoul<=45)
   lettre='N';
 if (numBoul>=46 && numBoul<=60)
   lettre='G';
 if (numBoul>=61 && numBoul<=75)
   lettre='O';
 document.write('<br> La Boule tiree est'+lettre+numBoul);
 </script>