在您的代码中将等于更改为 == ，它可以

Change equals to == in your code and it works DEMO

var elem = document.getElementById('userType');
elem.onchange = function() {
  var studentDiv = document.getElementById('student');
  var teacherDiv = document.getElementById('teacher');
  studentDiv.style.display = (this.value == "student") ? 'block' : 'none';
  teacherDiv.style.display = (this.value == "teacher") ? 'block' : 'none';
};