text: console.log("////////////////////////////// QUESTION 5.1 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\")
  
  // you have a unsorted array with just ONE missing number, build a function that returns what the missing number is
  
  
  function missingNumber(arr){
      // =========== code starts here =============
      var lengthPlus = arr.length+1, 
      sum = 0,
      expectedSum = lengthPlus * (lengthPlus + 1)/2;
      
      for(var i = 0, len = arr.length; i < len; i++){
        sum += arr[i];
      }
      
      return expectedSum - sum;
      // =========== code ends here ==============
    }
    
  
  
  missingNumber([5, 2, 4, 6, 1, 3, 8]);