JSFiddle - React, Tailwind, and code Playground

HTML

<select name="fruitOrVeg">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
</select>
<br />
<input type="text" name="nrOfFruit" />
<br />
<input type="button" id="submit_button" value="Submit" />

JavaScript

function calc() {
   var total,
       fruitOrVeg,
       nrOfFruit;

   fruitOrVeg = document.getElementsByName("fruitOrVeg")[0].value;
   nrOfFruit = document.getElementsByName("nrOfFruit")[0].value;

   total = fruitOrVeg * nrOfFruit;

   alert(total);
}

window.onload = function () {
    document.getElementById("submit_button").onclick = calc;
};