Nav Underline Effect

コード

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  background: #0f172a;
  color: #e2e8f0;
  font-family: system-ui, sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
}

.nav {
  display: flex;
  gap: 0.5rem;
  position: relative;
  padding-bottom: 4px;
}

.nav-link {
  padding: 0.5rem 1rem;
  color: #94a3b8;
  text-decoration: none;
  border-radius: 6px;
  transition: color 0.2s;
}

.nav-link:hover {
  color: #e2e8f0;
}

.nav-link.active {
  color: #3b82f6;
}

.nav-underline {
  position: absolute;
  bottom: 0;
  left: 0;
  height: 2px;
  background: #3b82f6;
  border-radius: 1px;
  transition: width 0.25s ease, left 0.25s ease;
}

const nav = document.querySelector(".nav");
const links = document.querySelectorAll(".nav-link");
const underline = document.querySelector(".nav-underline");

function setUnderline(link) {
  if (!link || !underline) return;
  links.forEach((l) => l.classList.remove("active"));
  link.classList.add("active");
  underline.style.width = `${link.offsetWidth}px`;
  underline.style.left = `${link.offsetLeft}px`;
}

links.forEach((link) => {
  link.addEventListener("click", (e) => {
    e.preventDefault();
    setUnderline(link);
  });
});

if (links.length) setUnderline(links[0]);

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Nav Underline Effect</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <nav class="nav" role="tablist">
    <a href="#" class="nav-link active" data-tab="0">Home</a>
    <a href="#" class="nav-link" data-tab="1">About</a>
    <a href="#" class="nav-link" data-tab="2">Contact</a>
    <span class="nav-underline" aria-hidden="true"></span>
  </nav>
  <script src="script.js"></script>
</body>
</html>

Animated underline that follows the active link. Reference patterns from tabs-vertical and breadcrumb-nav.

Features

CSS transition for underline position/width
Single moving indicator or per-link underline
No dependencies