To get all the class names present in a DOM element, you can use the classList property on the corresponding element using JavaScript.

Let's say you have an h1 element tag with class names text-red and rounded-border like this,

PS: I'm not writing the CSS for text-red and rounded-border classes since it is not needed to understand this property.

<h1 class="text-red rounded-border">Heading</h1>

Now let's get theses class names in JavaScript using the classList property like this,

// first get the reference to the h1 tag
const h1 = document.querySelector("h1");

// using classList property
const h1ClassNames = h1.classList;

console.log(h1ClassNames);

• The classList property returns the class names as DOMTokenList array-like object.
• You can get each class names from the object using the index as they are appearing in the element. For example, to get the first class name in the h1 element, you can use h1ClassNames[0].
• The DOMTokenList object is iterable so that you can use any valid looping construct in JavaScript.

See live example on JSBin.

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