text: // is “a” substring of “b”
  function isSubstring(a, b) {
      return b.includes(a);
  }
  
  // s1: waterbottle
  // s2: erbottlewat
  // check if s2 is a rotation of s1
  // using one call to isSubstring
  
  function isStringRotation(s1, s2) {
      const doubleStr = s2 + s2;
      return isSubstring(s1, doubleStr);
  }
  
  console.log(isStringRotation("waterbottle", "erbottlewat"))