如何让OOP Javascript记住以前选择的按钮？

function ButtonSelection() {
  this.selectedButton = null;
}

ButtonSelection.prototype.setSelectedButton = function(button) {
  this.selectedButton = button;
}

ButtonSelection.prototype.getSelectedButton = function() {
  return this.selectedButton;
}

var buttonSelection = new ButtonSelection();

var buttons = document.querySelectorAll('.button');

buttons.forEach(function(button) {
  button.addEventListener('click', function() {
    buttonSelection.setSelectedButton(button);
  });
});

var previousButton = buttonSelection.getSelectedButton();