TL;DR: Don't leave commented code. Remove it.

Problems

Readability
Dead Code
Lack of Coverage
Lack of Source Version Control

Solutions

Remove commented code
Implement Source Version Control

Context

When debugging code we tend to comment on code to see what happens.

As a final step, after all our tests pass, we must remove them following clean code practices.

Sample Code

Wrong

function arabicToRoman(num) {
  var decimal = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
  var roman = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
  var result = '';

  for(var i = 0; i < decimal.length; i++) {
    // print(i)
    while(num >= decimal[i]) {
      result += roman[i];
      num -= decimal[i];
    }    
  }
  // if (result > 0 return ' ' += result)

 return result;
}

Right

function arabicToRoman(arabicNumber) {
  var decimal = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
  var roman = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
  var romanString = '';

  for(var i = 0; i < decimal.length; i++) {
    while(arabicNumber >= decimal[i]) {
      romanString += roman[i];
      num -= decimal[i];
    }    
  }

 return romanString;
}