text: // functions type
  function add(n1, n2) {
      return n1 + n2;
  }
  function printResult(num) {
      console.log('Result: ' + num);
  }
  printResult(add(5, 15));
  var combineValue;
  combineValue = add;
  // combineValue = 2
  console.log(combineValue(5, 18));
  // call back function
  function addAndHandle(n1, n2, cb) {
      var result = n1 + n2;
      cb(result);
  }
  addAndHandle(10, 20, function (result) {
      console.log(result);
  });
  