Help with Smallest common multiple

1 
function smallestCommons(arr) {
  var val;
  
  arr = arr.sort(function(a, b){
    return a - b;
  });
  
  
  
  for(var num=1;;num++)
    {
  val=lcm(num,arr[0],arr[1]);
      if(val===true)
        break;
    }
  
  function lcm(n,a,b)
  {
  for(var i=a;i<=b;i++)
    {
      if(n%i!==0)
        return false;
                     
    }
  return true;
  }

I first sorted the array smallest to largest, then kept an infinite loop passing no’s from 1 to n to find the number ie.lcm of array of no’s,which will stop once the number is found, then a function to find lcm of all no’s between that array. but its not working for 1,13 and 18,23 while the logic is correct… Thanks in advance.