Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

JavaScript Interacting with the DOM Responding to Events Listening for Events with addEventListener()

S C
S C
5,947 Points

Why is "event" passed into the anonymous function for the copy event example?

Around 3:20, for the example showing the event listener on the copy event, why does he pass "event" into the anonymous function here, but not in the other examples?

// Example 1

document.addEventListener('copy', (event) => { alert('Please be sure to credit the author.') });

// Example 2

listItem.addEventListener('mouseover', () => { listItem.textContent = listItem.textContent.toUpperCase(); });

1 Answer

Steven Parker
Steven Parker
231,172 Points

The system always passes the event object to the handler when calling it, but defining a parameter for it is optional if your handler code doesn't use it.

Neither of the examples shown here need it, so it doesn't matter if the parameter is defined or not. My personal "good practice" preference is to always define it, but name the parameter "unused" when it isn't needed in the handler code.

document.addEventListener('click', (unused) => { alert("The event object wasn't needed.") });
S C
S C
5,947 Points

Perfect—Thank you!