containsLocation()

This example draws a red circle when the clicked point falls outside of the specified polygon (the Bermuda Triangle), and a blue triangle when the click falls inside the polygon.

Read the documentation.

// This example requires the Geometry library. Include the libraries=geometry
// parameter when you first load the API. For example:
// <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=geometry">

function initMap() {
  var map = new google.maps.Map(document.getElementById('map'), {
    center: {lat: 24.886, lng: -70.269},
    zoom: 5,
  });

  var triangleCoords = [
    {lat: 25.774, lng: -80.19},
    {lat: 18.466, lng: -66.118},
    {lat: 32.321, lng: -64.757}
  ];

  var bermudaTriangle = new google.maps.Polygon({paths: triangleCoords});

  google.maps.event.addListener(map, 'click', function(e) {
    var resultColor =
        google.maps.geometry.poly.containsLocation(e.latLng, bermudaTriangle) ?
        'blue' :
        'red';

    var resultPath =
        google.maps.geometry.poly.containsLocation(e.latLng, bermudaTriangle) ?
        // A triangle.
        "m 0 -1 l 1 2 -2 0 z" :
        google.maps.SymbolPath.CIRCLE;

    new google.maps.Marker({
      position: e.latLng,
      map: map,
      icon: {
        path: resultPath,
        fillColor: resultColor,
        fillOpacity: .2,
        strokeColor: 'white',
        strokeWeight: .5,
        scale: 10
      }
    });
  });
}
<div id="map"></div>
/* Always set the map height explicitly to define the size of the div
 * element that contains the map. */
#map {
  height: 100%;
}
/* Optional: Makes the sample page fill the window. */
html, body {
  height: 100%;
  margin: 0;
  padding: 0;
}
<!-- Replace the value of the key parameter with your own API key. -->
<script src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk&libraries=geometry&callback=initMap"
         async defer></script>

Try it yourself

You can experiment with this code in JSFiddle by clicking the <> icon in the top-right corner of the code window.

<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no">
    <meta charset="utf-8">
    <title>containsLocation()</title>
    <style>
      /* Always set the map height explicitly to define the size of the div
       * element that contains the map. */
      #map {
        height: 100%;
      }
      /* Optional: Makes the sample page fill the window. */
      html, body {
        height: 100%;
        margin: 0;
        padding: 0;
      }
    </style>
  </head>
  <body>
    <div id="map"></div>
    <script>
      // This example requires the Geometry library. Include the libraries=geometry
      // parameter when you first load the API. For example:
      // <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=geometry">

      function initMap() {
        var map = new google.maps.Map(document.getElementById('map'), {
          center: {lat: 24.886, lng: -70.269},
          zoom: 5,
        });

        var triangleCoords = [
          {lat: 25.774, lng: -80.19},
          {lat: 18.466, lng: -66.118},
          {lat: 32.321, lng: -64.757}
        ];

        var bermudaTriangle = new google.maps.Polygon({paths: triangleCoords});

        google.maps.event.addListener(map, 'click', function(e) {
          var resultColor =
              google.maps.geometry.poly.containsLocation(e.latLng, bermudaTriangle) ?
              'blue' :
              'red';

          var resultPath =
              google.maps.geometry.poly.containsLocation(e.latLng, bermudaTriangle) ?
              // A triangle.
              "m 0 -1 l 1 2 -2 0 z" :
              google.maps.SymbolPath.CIRCLE;

          new google.maps.Marker({
            position: e.latLng,
            map: map,
            icon: {
              path: resultPath,
              fillColor: resultColor,
              fillOpacity: .2,
              strokeColor: 'white',
              strokeWeight: .5,
              scale: 10
            }
          });
        });
      }
    </script>
    <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=geometry&callback=initMap"
         async defer></script>
  </body>
</html>