如何从另一个选择中获取上一个选择的值到当前选择onclick中？

var previousValue;

function onChangeHandler() {
  previousValue = document.getElementById("previousSelect").value;
}

function onClickHandler() {
  var currentValue = document.getElementById("currentSelect").value;
  console.log("Previous value: " + previousValue);
  console.log("Current value: " + currentValue);
}

function onChangeHandler() {
  var previousValue = document.getElementById("previousSelect").value;
  document.getElementById("currentSelect").setAttribute("data-previous-value", previousValue);
}

function onClickHandler() {
  var currentValue = document.getElementById("currentSelect").value;
  var previousValue = document.getElementById("currentSelect").getAttribute("data-previous-value");
  console.log("Previous value: " + previousValue);
  console.log("Current value: " + currentValue);
}

function createOnChangeHandler() {
  var previousValue;

  return function() {
    previousValue = document.getElementById("previousSelect").value;
  };
}

var onChangeHandler = createOnChangeHandler();

function onClickHandler() {
  var currentValue = document.getElementById("currentSelect").value;
  console.log("Previous value: " + onChangeHandler.previousValue);
  console.log("Current value: " + currentValue);
}