JSFiddle - React, Tailwind, and code Playground

HTML

<form id="aggregateForm">
    <input id="inputNum" value="" type="text"><input id="addnum" value="Add" onclick="add();" type="button">
    <br>
    <select id="selectBox" name="select" size="15" style="width:190px;">  

    </select>
    <br>
    <input id="calculate" value="Calculate" onclick="calculates();" type="button">
    <input id="reset" value="Reset" onclick="reset();" type="button">
</form>

JavaScript

var selectbox = document.getElementById("selectBox");

function add() {
    //Function to add a new number to the list of digits
    //Parses an integer to ensure everything works okay
    if (IsNumeric(document.getElementById("inputNum").value) == true) {
        selectbox.options[selectbox.options.length] = new Option(document.getElementById("inputNum").value, document.getElementById("inputNum").value);
        inputNum.focus();
    }
    else if (IsNumeric(document.getElementById("inputNum").value) == false) {
        alert("I'm sorry, but you have entered an invalid number. Please enter a number into the input box.");
        inputNum.focus();
    }
}

//Calculate all numbers
var x = [];

function calculates() {
    for (var i = 0; i < selectbox.options.length; i++) {
        x[i] = selectbox.options[i].value;
        alert(x[i]);
    }


}

//IsNumeric function coding taken from http://stackoverflow.com/questions/18082/validate-numbers-in-javascript-isnumeric, code by Joel Coehoorn


function IsNumeric(input) {
    return (input - 0) == input && input.length > 0;
}