classList API

The classList property exposes a live DOMTokenList that lets you manipulate an element's class attribute without touching the full string.

const card = document.querySelector('.card');

card.classList.add('active');          // add one class
card.classList.add('highlighted', 'expanded'); // add multiple
card.classList.remove('disabled');     // remove one
card.classList.toggle('open');         // add if absent, remove if present
console.log(card.classList.contains('active')); // true / false
card.classList.replace('old-class', 'new-class');

toggle with a Boolean

Pass a second boolean argument to force-add or force-remove regardless of current state:

// Force-add 'hidden' when isHidden is true, remove when false
el.classList.toggle('hidden', isHidden);

Why Classes Instead of Inline Styles?

Write the visual state once in CSS (.active { background: #333 }) and toggle the class from JavaScript. Changing a CSS variable in one place then updates every element that uses it - something inline styles cannot do cleanly.