Here's an example from the google maps api page:
<!DOCTYPE html>
<html>
<head>
<!-- This stylesheet contains specific styles for displaying the map
on this page. Replace it with your own styles as described in the
documentation:
https://developers.google.com/maps/documentation/javascript/tutorial -->
<link rel="stylesheet" href="/maps/documentation/javascript/demos/demos.css">
</head>
<body>
<div id="map"></div>
<script>
function initMap() {
var chicago = {lat: 41.85, lng: -87.65};
var indianapolis = {lat: 39.79, lng: -86.14};
var map = new google.maps.Map(document.getElementById('map'), {
center: chicago,
scrollwheel: false,
zoom: 7
});
var directionsDisplay = new google.maps.DirectionsRenderer({
map: map
});
// Set destination, origin and travel mode.
var request = {
destination: indianapolis,
origin: chicago,
travelMode: 'DRIVING'
};
// Pass the directions request to the directions service.
var directionsService = new google.maps.DirectionsService();
directionsService.route(request, function(response, status) {
if (status == 'OK') {
// Display the route on the map.
directionsDisplay.setDirections(response);
}
});
}
</script>
<script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap"asyncdefer></script></body></html>
Reference