Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form name="form">
        <button id="addCountry">Add county list</button>
    </form>

    <script>
        const addCountry = document.querySelector('#addCountry');
        const form = document.forms.form;
        let currentCountry = '';

        function getSelectedCountries() {
            return Array.from(form.countries).map(country => country.value);
        }

        function handleChangeCountry(event) {
            const element = event.target;

            if (element.nodeName === 'SELECT') {
                const countries = getSelectedCountries();

                if (countries.includes(currentCountry)) {
                    alert('Country already selected');

                    element.value = currentCountry;
                }
            }
        }

        function handleFocusCountry(event) {
            const element = event.target;

            if (element.nodeName === 'SELECT') {
                currentCountry = element.value;
            }
        }

        function handleClickButton(event) {
            event.preventDefault();

            const countryList = ['Nicaragua', 'Costa rica', 'Honduras', 'El salvador', 'Guatemala', 'Panama'];
            const select = document.createElement('select')
            const form = document.forms.form;

            select.name = 'countries';
            select.id = 'countries';

            countryList.forEach(country => {
                const option = new Option(country, country)

                select.appendChild(option);
            });

            form.appendChild(select);
        }

        addCountry.addEventListener('click', handleClickButton);
        form.addEventListener('change', handleChangeCountry);
        form.addEventListener('focus', handleFocusCountry);
    </script>
</body>
</html>