JavaScript ES2015 new stuff

I am finding this example hard to read as in what happens first...

'use strict';

var Teacher = function (data) { this.name = data.name; this.greet = function (studentCnt) { console.log(studentCnt); let promise = new Promise(function (resolve, reject) { if (studentCnt === 0) { reject('Zero students in class'); } else { resolve(Hello to ${studentCnt} of my favorite students!); } }); return promise; } }

var Classroom = function (data) { this.subject = data.name; this.teacher = data.teacher; this.students = []; this.addStudent = function (data) { this.students.push(data); this.greet(); } this.greet = () => { this.teacher.greet(this.students.length).then( (function (classroom) { return function (greeting) { console.log(${classroom.teacher.name} says:, greeting); } })(this), function (err) { console.log(err); }) } }

var myTeacher = new Teacher({ name: 'James' }); var myClassroom = new Classroom({ name: 'The Future of JavaScript', teacher: myTeacher });

myClassroom.addStudent({ name: 'Dave' });

It looks like to me the first line is var myTeacher = new Teacher({ name: 'James' });

If this is the case then the Teacher object will be created

but this.greet = function (studentCnt)

studentCnt has no value...