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JavaScript JavaScript and the DOM (Retiring) Responding to User Interaction Event Delegation

Posted by Dorota Nowak
Dorota Nowak
12,060 Points

Problem with task.

Hi there :) I have problem with this task: Currently, the event listener applies a yellow background color to the section element and its child elements when clicked. Add a condition that changes the background of the <input> elements only.

I get this message: There was an error with your code: TypeError: 'undefined' is not a function (evaluating 'section.querySelector('button')')

What do I do wrong? Why am I getting this error?

app.js 
let section = document.getElementsByTagName('section')[0].getElementsByTagName('input');


for (let i = 0; i < section.length; i++) {
section[i].addEventListener('click', (e) => {

  e.target.style.backgroundColor = 'rgb(255, 255, 0)';

});
};
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>JavaScript is an exciting language that you can use to power web servers, create desktop programs, and even control robots. But JavaScript got its start in the browser way back in 1995.</p>
            <hr>
            <p>Things to Learn</p>
            <ul>
                <li>Item One: <input type="text"></li>
                <li>Item Two: <input type="text"></li>
                <li>Item Three: <input type="text"></li>
                <li>Item Four: <input type="text"></li>
            </ul>
            <button>Save</button>
        </section>
        <script src="app.js"></script>
    </body>
</html>

3 Answers

Peter Vann
Peter Vann
36,425 Points

Hi Dorota!

Just remember that if the instructions say "add a condition", you will need an if statement/block somewhere.

This passes:

let section = document.getElementsByTagName('section')[0];

section.addEventListener('click', (e) => {
    if(e.target.tagName == "INPUT") {  // Test if current element is an input element
        e.target.style.backgroundColor = 'rgb(255, 255, 0)';
    }
});

Keep in mind that by using "getElementsByTagName('section')[0];", this only applies to the first section on the page.

Also, keep in mind that e.target.tagName is all uppercase.

Also, make sure to use == (comparison operator) and not = (assignment operator). I make that mistake on occasion, as do others. It's an easy one to make (and sometimes difficult to debug).

I hope that helps.

Stay safe and happy coding!

Dorota Nowak
Dorota Nowak
12,060 Points
Dorota Nowak
Dorota Nowak
12,060 Points

Now I see what was the problem. Thank you very much for your help!

Michael Hicks
Michael Hicks
23,977 Points
Michael Hicks
Michael Hicks
23,977 Points

The code below worked for me, but why does 'INPUT' have to be capitalized? Do all event object attributes need to be capitalized or just with this objective? I don't see any reference in the code where toUpperCase() is used. Thank you.

let section = document.getElementsByTagName('section')[0];
section.addEventListener('click', (e) => {
  if (e.target.tagName == 'INPUT') {
    e.target.style.backgroundColor = 'rgb(255, 255, 0)';
  };
});