[HTML 5] Event Delegation and Dynamic Events

For example, we have a list of 'li', for each of them we want to attach event listener to it

Bad approach:

const app = document.getElementById('app');
app.innerHTML = `
  <h1>JavaScript DOM</h1>
  <ul id="list">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
  </ul>
`;

const button = document.querySelector('button');
const list = document.querySelector('#list');

const items = [...list.querySelectorAll('li')];
 items.forEach(item => {
   item.addEventListener('click', handleClick);
 });

Here, we use 'forEach' on 'li's to attach event listener.

The problem for that approach is: when we want to dynamiclly add a new 'li' into the list, that new 'li' won't get the event listener dynamiclly.

const app = document.getElementById('app');
app.innerHTML = `
  <h1>JavaScript DOM</h1>
  <ul id="list">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
  </ul>
`;

const button = document.querySelector('button');
const list = document.querySelector('#list');

const items = [...list.querySelectorAll('li')];
 items.forEach(item => {
   item.addEventListener('click', handleClick);
 });


const item5 = document.createElement('li')
item5.text = 'Item 5'
list.append('item')

No effect on clicking 'Item 5'.

To solve the problem, we can use Event Delegation partten.

Instead of add event listener on 'li' element, we add it to parent element 'ul'. Then rely on event bubbling up, we capture event in 'ul'. 

const app = document.getElementById('app');
app.innerHTML = `
  <h1>JavaScript DOM</h1>
  <button type="button">
    Add Item
  </button>
  <ul id="list">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
  </ul>
`;

const button = document.querySelector('button');
const list = document.querySelector('#list');

// const items = [...list.querySelectorAll('li')];
// items.forEach(item => {
//   item.addEventListener('click', handleClick);
// });

function handleClick(event) {
  if (event.target.nodeName.toLowerCase() !== 'li') {
    return;
  }
  console.log(event.target.innerText);
}

list.addEventListener('click', handleClick);

button.addEventListener('click', () => {
  const items = list.querySelectorAll('li');
  const li = document.createElement('li');
  li.innerText = `Item ${items.length + 1}`;
  list.append(li);
});