Attaching handlers to new elements in JavaScript

Suppose we have the list ul and a button:

<ul> <li>item</li> <li>item</li> <li>item</li> <li>item</li> <li>item</li> </ul> <button>add</button>

Let's get our elements into the corresponding variables:

let button = document.querySelector('button'); let list = document.querySelector('ul'); let items = list.querySelectorAll('li');

Let's make it so that when you click on any li, an exclamation mark is added to the end of it:

for (let item of items) { item.addEventListener('click', function() { this.innerHTML = this.innerHTML + '!'; }); }

Let's now make it so that when the button is pressed, a new li is added to the end of the list:

button.addEventListener('click', function() { let item = document.createElement('li'); item.innerHTML = 'item'; list.appendChild(item); });

We will however get a problem: clicking on the newly added li will not add an exclamation mark to the end. The point is that we add a click handler only to those li that originally existed, but not to new ones.

Let's fix the problem by attaching a click handler to the new li:

button.addEventListener('click', function() { let item = document.createElement('li'); item.innerHTML = 'item'; item.addEventListener('click', function() { // click handler this.innerHTML = this.innerHTML + '!'; }); list.appendChild(item); });

However, now the handler function code is duplicated in two places - for the original li and for the new ones. Let's fix this by moving it to a separate function:

function handler() { this.innerHTML = this.innerHTML + '!'; }

Use our function to avoid code duplication:

for (let item of items) { item.addEventListener('click', handler); } button.addEventListener('click', function() { let item = document.createElement('li'); item.innerHTML = 'item'; item.addEventListener('click', handler); list.appendChild(item); });

The task is generally solved and we have avoided duplicating the code of the handler function. However, we still have to hang event handlers in two places: both in the loop for the existing li, and when the button is clicked. In the next lesson, we will look at a way to get rid of this inconvenience.

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