underscore-2.4.js

var studentsByClass = _.groupBy(students, function (student) { return student.class })
/* {
  "A": [
    { name: "John", grades: [7.0, 6.9, 8.0], class: "A" },
    { name: "Paul", grades: [5.0, 8.7, 3.7], class: "A" }
  ],
  "B": [
    { name: "Luke", grades: [9.0, 8.2, 6.2], class: "B" },
    { name: "Mark", grades: [6.0, 9.0, 8.4], class: "B" }
  ]
} */