script.js

function throttle(func, time) {
    clearTimeout(func._timeout);
    func._timeout=setTimeout(() => { func.call(); }, time);
}

function handle_date_change(e) {
    async function _handle_date_change() {
        var response = await fetch('/date?date=' + encodeURIComponent(e.target.value));
        var response_json = await response.json();
        format_strings = '';
        for (entry in response_json) {
            format_strings += '<p>' + response_json[entry] + '</p>';
        }
        document.querySelector('#format').innerHTML = format_strings;
    }
    throttle((e) => { _handle_date_change(e); }, 100);
}

var date_input = document.querySelector('#date');
date_input.addEventListener('focus', handle_date_change, false);
date_input.addEventListener('change', handle_date_change, false);
date_input.addEventListener('keydown', handle_date_change, false);
date_input.focus();