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JavaScript JavaScript and the DOM (Retiring) Traversing the DOM Sibling Traversal

Abua KC
Abua KC
5,932 Points
Posted by Abua KC
Abua KC
5,932 Points

There was an error with your code: TypeError: 'null' is not an object (evaluating 'ul.className')

There was an error with your code: TypeError: 'null' is not an object (evaluating 'ul.className')

app.js 
const list = document.getElementsByTagName('ul')[0];

list.addEventListener('click', function(e) {
  if (e.target.tagName == 'BUTTON') {
    let li = e.target.parentNode;
    let ul = li.previousElementSibling;
    ul.className.add('p','highlight');
    }
});
index.html 
<!DOCTYPE html>
<html>
    <head>
        <title>JavaScript and the DOM</title>
    </head>
    <link rel="stylesheet" href="style.css" />
    <body>
        <section>
            <h1>Making a Webpage Interactive</h1>
            <p>Things to Learn</p>
            <ul>
                <li><p>Element Selection</p><button>Highlight</button></li>
                <li><p>Events</p><button>Highlight</button></li>
                <li><p>Event Listening</p><button>Highlight</button></li>
                <li><p>DOM Traversal</p><button>Highlight</button></li>
            </ul>
        </section>
        <script src="app.js"></script>
    </body>
</html>

2 Answers

Katie Wood
Katie Wood
19,141 Points

Hi there,

You're close - first, we can simplify things a little bit. We really just need to select the element right before the event target - I'm going to call it 'p' here because it's a paragraph:

const list = document.getElementsByTagName('ul')[0];

list.addEventListener('click', function(e) {
  if (e.target.tagName == 'BUTTON') {
    let p = e.target.previousElementSibling;
    p.className.add('p','highlight');
    }
});

Now, we get to the reason for the error. There are two ways to do this - one uses className and the other uses classList. You can do either one, but .add is used with classList. The simplest solution (closest to your current code) would be to change className to classList, like so:

p.classList.add('highlight');

But you could also do it like:

p.className += 'highlight';

The whole thing would look like this:

const list = document.getElementsByTagName('ul')[0];

list.addEventListener('click', function(e) {
  if (e.target.tagName == 'BUTTON') {
    let p = e.target.previousElementSibling;
    p.classList.add('highlight');
    }
});

Hope this helps!

Abua KC
Abua KC
5,932 Points
Abua KC
Abua KC
5,932 Points

Thank you

Katie Wood
Katie Wood
19,141 Points

No problem!