Cómo saber si se ha seleccionado la entrada SELECT en JS

// See the below HTML first
/*
<select id="cardtype" name="cards">
    <option value="mastercard">Mastercard</option>
    <option value="maestro">Maestro</option>
    <option value="visadebit">Visa Debit</option>
</select>
*/

// Javascript Solution
var select = document.getElementById("cardtype");
var selectedValue = select.options[select.selectedIndex].value;

// Courtesy: Sachin - https://stackoverflow.com/users/1041642/sachin
Horny Dev