Value Objects: Implementation

grade.js

var _ = require('underscore');

var Grade = function(percentage) {
  this.percentage = percentage;
  this.grade = this.grade(percentage); 
};

Grade.prototype = _.extend(Grade.prototype, {

  passingGradeLetters: function() {
    return _.chain(Grade.grades).where({passing: true}).pluck('letter').value();
  },

  grade: function(percentage) {
    return _.find(Grade.grades, function(grade) { 
      return percentage >= grade.minimumPercentage; 
    });
  },

  letterGrade: function() {
    return this.grade.letter;
  },

  isPassing: function() {
    return this.grade.passing
  },

  isImprovementFrom: function(grade) {
    return this.isBetterThan(grade);
  },

  isBetterThan: function(grade) {
    return this.percentage > grade.percentage;
  },

  valueOf: function() {
    return this.percentage;
  }

});

Grade.grades = [
  {letter: 'A', minimumPercentage: 0.9, passing: true},
  {letter: 'B', minimumPercentage: 0.8, passing: true},
  {letter: 'C', minimumPercentage: 0.7, passing: true},
  {letter: 'D', minimumPercentage: 0.6, passing: true},
  {letter: 'F', minimumPercentage: 0,   passing: false}
];

module.exports = Grade;