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JavaScript Callback Functions in JavaScript Callbacks and the DOM Using Callbacks on Text Inputs

Brian Anstett
Brian Anstett
5,831 Points
Posted by Brian Anstett
Brian Anstett
5,831 Points

Function declaration for callback instead of an arrow function.

When I declare a function to be used as the callback before the addEventListener, I do not get the correct results. I must be missing something in my understanding because I can't see why the following code wouldn't work.

const nameInput = document.getElementById('name'); const messageTextArea = document.getElementById('message');

function focus(e){ e.target.className = 'highlight'; }

nameInput.addEventListener('focus', focus(event)); nameInput.addEventListener('blur', event => event.target.className = '');

NOTE: I get the correct results when using an arrow function as the callback.

const nameInput = document.getElementById('name'); const messageTextArea = document.getElementById('message');

function focus(e){ e.target.className = 'highlight'; }

nameInput.addEventListener('focus', event => event.target.className = 'highlight'); nameInput.addEventListener('blur', event => event.target.className = '');

Brian Anstett
Brian Anstett
5,831 Points
Brian Anstett
Brian Anstett
5,831 Points

After watching the next video, I was reminded I just need the name of the callback function. So it was unnecessary of me to provide "(event)".

1 Answer

Steven Parker
Steven Parker
229,732 Points
Steven Parker
Steven Parker
229,732 Points

It's not just unnecessary, it causes an error. When you put parentheses after a function name (with or without arguments), you are invoking ("calling") the function immediately and passing the function's return value as the argument.

But for a callback, you want to pass the function itself, so it's important to not use the parentheses.