How to toggle CSS class on tag using vanillaJS - No JQuery

Question

Is it possible to apply a CSS class to the <body> tag of an HTML document?

I have so far been attempting something as such:

document.getElementsByClassName("body").add("cssClass");

The hope is to apply a filter over the entire DOM as to darken the page at specific times, but keep all functionality of the DOM itself.

sinisake · Accepted Answer

Or, shorter: document.body.className = 'classname';

As suggested by Gaby, to avoid replacing of already applied class(es):

document.body.classList.add('classname')

How to toggle CSS class on <body> tag using vanillaJS - No JQuery