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 Callback Functions in JavaScript Callbacks and the DOM Using the Same Callback on Multiple Elements

Posted by juami alcazar
juami alcazar
6,443 Points

just to make sure, the usage of event in the arrow function is just a parameter/argument right? or is it required

const blurHandler = event => { event.target.className = 'highlight'; }

1 Answer

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

The identifier "event" is a parameter, but it's also necessary in this case because it is used in the handler code.

Arrow functions can also be written without a parameter by placing empty parentheses where the parameter would otherwise go.

const warn = () => { alert("Watch out!"); };
Tom Porter
Tom Porter
5,589 Points
Tom Porter
Tom Porter
5,589 Points

Just in case this is of help to anyone else. This was the first video I'd come across where it used an arrow function without the parentheses around the parameters (here it uses 'event' as a parameter without any parentheses), and I wasn't sure why they were omitted here but previously the standard notation seemed to be:

const exampleVariable = () => { console.log ("example"); };

...I found the page below which confirmed that for arrow notation the parentheses around the parameter are optional when there is exactly one parameter. So you could still use parentheses around 'event' if you wish, or omit. For zero, two or more parameters you must still use parentheses.

https://eslint.org/docs/latest/rules/arrow-parens#:~:text=Arrow%20functions%20can%20omit%20parentheses,of%20parentheses%20in%20arrow%20functions.