index.js

var txtNumber = document.getElementById('txtNumber');
var txtInterval = document.getElementById('txtInterval');
var btnGenerate = document.getElementById('btnGenerate');
var ans = document.getElementById('ans');

document.addEventListener('DOMContentLoaded', onFocus);
btnGenerate.addEventListener('click', onClick);
txtInterval.addEventListener('keydown', onKeyDown);
txtNumber.addEventListener('keydown', onKeyDown);
 
function onClick() {
    let count = 0;
    if (!validateNumber(txtNumber.value)) {
        while (count <= txtInterval.value) {
            ans.innerHTML += parseFloat(txtNumber.value) + ' X ' + count + ' = ' + (parseFloat(txtNumber.value) * count) + '<br>';
            count++;
        }
        validateInterval(txtInterval.value);
    }
    return count;
}

function validateNumber(number) {
    let ret = false;
    if (number == '') ret = true;
    else if (number != '' && ans != '') ans.innerHTML = '';
    return ret;
}

function validateInterval(interval) {
    let ret = false;
    if (interval > 100) {
        ans.innerHTML = '';
        ret = true;
    }
    return ret;
}

function onKeyDown(e) {
    if (e.key === 'e') e.preventDefault();
}

function onFocus() {
    txtNumber.focus();
}