navigator.geolocation.getCurrentPosition(success, error) requests the device's coordinates. The browser shows a permission prompt; if the user allows, your success callback gets a position with coords.latitude and coords.longitude.
It can fail or be denied, so always pass an error callback. Use watchPosition for live tracking (and clearWatch to stop). Geolocation only works over HTTPS (or localhost) for privacy.
<!doctype html>
<html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:sans-serif;padding:20px">
<button id="b" style="font:inherit;color:#b388ff;background:none;border:1px solid #b388ff;padding:8px 14px;border-radius:3px">find me</button>
<pre id="out" style="margin-top:12px"></pre>
<script>
document.getElementById('b').onclick = () => {
const out = document.getElementById('out');
if(!('geolocation' in navigator)) return out.textContent = 'no geolocation here';
navigator.geolocation.getCurrentPosition(
p => out.textContent = 'lat ' + p.coords.latitude.toFixed(3) + ' · lon ' + p.coords.longitude.toFixed(3),
e => out.textContent = 'denied or unavailable: ' + e.message
);
};
</script>
</body></html>