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

Posted by Brendan Dzeguze
.a{fill-rule:evenodd;}techdegree seal-36
Brendan Dzeguze
Front End Web Development Techdegree Graduate 14,567 Points

My code isn't running. can someone help?

The terminal says 'undefined' is not an object.

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

list.addEventListener('click', function(e) {
  if (e.target.tagName == 'BUTTON') {
  e.previousElementSibling.add('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>
John Johnson
John Johnson
11,790 Points

Two issues:

  1. You're selecting e, not the target of e. In this case, 'e' is the click event itself, but the 'target' property of 'e' is the object you clicked on.
  2. The add() method isn't going to give you what you want here. JavaScript has a built in property for class names: className. You'll want to set that equal to 'highlight' something like this:
object.className = 'highlight';

1 Answer

Andres Sanchez Martinez
Andres Sanchez Martinez
1,411 Points
Andres Sanchez Martinez
Andres Sanchez Martinez
1,411 Points

Hello,

I ran the code on my browser and it says: "Cannot read property 'add' of undefined at HTMLUListElement.<anonymous>"

I would suggest checking this post as it seems to be already solved: https://teamtreehouse.com/community/how-to-add-a-class-to-the-previous-sibling-element

Greetings, Andres