How do I make a function that checks if an element has a certain class and if true, runs a separate function?

I’m making a site for me to experiment and go out of my comfort zone and I’m trying to add a button that runs a different function based on whether or not it’s in dark or light theme using javaScript. I want it to work like this:

if elementClass == "dark" 
run function 1
else if elementClass == "light" 
run function 2

This is my current code
html:
<button id="fullscreen" class="fullscreen dark" onclick='checkClass()'> Fullscreen </button>

script:

function checkClass() {
    if ($('.fullscreen').hasClass('dark') === true) {
        openFullscreenDark()    
    } else if ($('.fullscreen').hasClass('light') === true){
        openFullscreenLight()   
    }
}   

The openfullscreen functions are working, however I don’t know how to properly check for classes as well as run a function in a function.

The if statements I found in another post and I don’t know how to use jQuery.

Is this possible to do? If so how?

>Solution :

You can rewrite the checkClass() function with vanilla JS like this:

function checkClass() {
    if (document.querySelector('.fullscreen').classList.contains('dark') === true) {
        openFullscreenDark()    
    } else if (document.querySelector('.fullscreen').classList.contains('light') === true){
        openFullscreenLight()   
    }
} 

Since contains() returns a boolean, you can also simplify it to:

function checkClass() {
    if (document.querySelector('.fullscreen').classList.contains('dark')) {
        openFullscreenDark()    
    } else if (document.querySelector('.fullscreen').classList.contains('light')){
        openFullscreenLight()   
    }
} 

Since you are only checking one element, you could also use getElementById() like this:

function checkClass() {
    if (document.getElementById('fullscreen').classList.contains('dark')) {
        openFullscreenDark()    
    } else if (document.getElementById('fullscreen').classList.contains('light')){
        openFullscreenLight()   
    }
} 

As you can see, there are many different ways to do this in vanilla JS, but they all do the same thing.