JavaScript Value Objects using stampits (from http://crushlovely.com/journal/7-patterns-to-refactor-javascript-applications-value-objects/)

grade.js

const stampit = require('stampit')

Grade = stampit({
  init(percentage) {
    this.percentage = percentage
    this.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}
    ]
    this.grade = this.grade(percentage)
  },

  methods: {
    passingGradeLetters() {
      return this.grades.filter(x => x.passing).map(x => x.letter)
    },

    grade(percentage) {
      debugger
      return this.grades.find(x => percentage >= x.minimumPercentage)
    },

    letterGrade() {
      return this.grade.letter
    },

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

    isImprovementFrom(grade) {
      return this.isBetterThan(grade)
    },

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

    valueOf() {
      return this.percentage
    }
  }
})

module.exports = Grade