Element.className

The className property of the Element interface gets and sets the value of the class attribute of the specified element.

Value

A string variable representing the class or space-separated classes of the current element.

Examples

let el = document.getElementById('item');

if (el.className === 'active'){
  el.className = 'inactive';
} else {
  el.className = 'active';
}

Notes

The name className is used for this property instead of class because of conflicts with the "class" keyword in many languages which are used to manipulate the DOM.

className can also be an instance of SVGAnimatedString if the element is an SVGElement. It is better to get/set the className of an element using Element.getAttribute and Element.setAttribute if you are dealing with SVG elements. However, take into account that Element.getAttribute returns null instead of "" if the element has an empty class attribute. 

elm.setAttribute('class', elm.getAttribute('class'))

Note: The class is an HTML Attribute, while the className is a DOM Property.

Specifications

Specification
DOM Standard
# ref-for-dom-element-classname①

Browser compatibility

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See also