dropdown menu with selected value code example

Example 1: javascript get selected option

var e = document.getElementById("selectElementID");
var value=e.selectElement.options[e.selectedIndex].value;// get selected option value
var text=e.options[e.selectedIndex].text;//get the selected option text

Example 2: how to call a function when dropdown menu's value is selected in html

var menu = document.getElementById("change_chart");
menu.addEventListener("change", generateData);

function generateData(event) {
  if (menu.value == '1') {
    alert(1);
  } else if (menu.value == '2') {
    alert(2);
  } else if (menu.value == '3') {
    alert(3);
  }
}