function power_of2(num)
{
  var array=[];
  for(var index =1; index<20 ; index++)
  {

    array.push(Math.pow(2,index));
  }

  for(var index1 =0 ;index1<array.length; index1 ++)
  {
    if(array[index1] === num)
    { return "true" ;}

  }

  return "false";
}

console.log(power_of2(4));