var mymap = L.map('mapid').setView([51.505, -0.09], 13);
L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
attribution: 'Map data © <a href="https://www.openstreetmap.org/">OpenStreetMap</a> contributors, <a href="https://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="https://www.mapbox.com/">Mapbox</a>',
maxZoom: 18,
id: 'mapbox.streets',
accessToken: 'your.mapbox.access.token'
}).addTo(mymap);
var marker = L.marker([51.5, -0.09]).addTo(mymap);
marker.bindPopup("<b>Hello world!</b><br>I am a popup.").openPopup();
var popup = L.popup()
.setLatLng([51.5, -0.09])
.setContent("I am a standalone popup.")
.openOn(mymap);