Answers for "how event listener works in javascript"

event listener javascript

const element = document.querySelector(".class__name");

element.addEventListener("click", () => {
	console.log("clicked element");
});

event listener javascript

document.querySelector('div').addEventListener('click', () => {
  console.log('div clicked');
});

event listener javascript

//DOM Level 2 Event Listener (Advanced technique to call an event)
document.getElementById("btn2").addEventListener('click', () => {
    alert("hello world");
}, false);
//our third param of DOM Level 2 Event Listener
// false = event trigger from bottom to top(bubbling phase)
// true = event trigger from top to bottom (capturing phase)


//TRADITIONAL DOM EVENT HANDLERS (very old)
function show_alert() {
    alert("hello world");
}
document.getElementById("btn1").onclick = show_alert;

js eventlistener to add and remove stylings

const form = document.querySelector('form');
const fname = document.getElementById('fname');
const lname = document.getElementById('lname');
const para = document.querySelector('p');

form.onsubmit = function(e) {
  if (fname.value === '' || lname.value === '') {
    e.preventDefault();
    para.textContent = 'You need to fill in both names!';
  }
}