Cool letter grade to point value conversion

letter_grade_calc.js

/**
 * 
 * @param {string} letter the string representation of the grade. Examples: ["a-", "A", "F", "b+"]
 * @param {number} points the number of points that an perfect A represents on the scale
 * @param {number} inc the number of points in between each full letter grade
 */
function grade(letter = "", points = 4, inc = 1) {
  if (letter.length < 1 || letter.length > 2) {
    throw 'the length of the letter grade string must be equal to 1 or 2'
  }
  letter = letter.toUpperCase()

  let lettercode = letter.charCodeAt(0)
  let value = points - ((lettercode - 65) * inc)
  value = value < 0 ? 0 : value

  if (letter.length < 2) return value

  let modcode = letter.charCodeAt(1)
  let mod = 0.333 * (modcode == 43 ? 1 : -1)
  return value + mod
}

module.exports = grade