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 JavaScript and the DOM Getting a Handle on the DOM Select an Element by ID

Turandeep Munday
Turandeep Munday
6,047 Points

When i move the function out of the event listener itself my page loads with the styles already applied

// we have a button object with id="btn-main" and <h1 id ="headline">

const headline = document.getElementById('headline');
const selectButtonElement = document.getElementById('btn-main');

function changeBorder(selection, borderString) {
  let a = selection;
  let b = borderString;
  a.style.border = b;  
}; 

selectButtonElement.addEventListener('click', changeBorder(headline, 'solid 2px red'));

My thinking is when i select the button element then the fucntion should run with selected params and it should run much in the same way as using an arrow function as second argument on .addEventListener

Turandeep Munday
Turandeep Munday
6,047 Points

this is the code that works from the course

const headline = document.getElementById('headline');
const selectButtonElement = document.getElementById('btn-main');

function changeBorder(selection, borderString) {
  let a = selection;
  let b = borderString;
  a.style.border = b;  
}; 

selectButtonElement.addEventListener('click', () => {
                                     headline.style.border = 'solid 2px red';
                                     })

1 Answer

Steven Parker
Steven Parker
229,708 Points

When a function (like addEventListener) needs a callback argument, you should pass only a reference to the function. But what's happening in this code is that handler function is being called, and its return value (which is "undefined" in this case) is being passed as the callback. Calling the function is what applies the styles immediately.

In the original lesson code, the arrow function expression creates a reference and defines a function but the function isn't actually called until the event occurs.