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function fact(x) {
   if(x===0) {
      return 1;
   }
   return x * fact(x-1);
}

function fact(x) {
       if(x === 0) {
           return 1;
       }
       if(x < 0 ) {
           return undefined;
       }
       for(var i = x; --i; ) {
           x *= i;
       }
       return x;
}