Google Map API V3: How to add Custom data to markers

As a Google Marker is a JavaScript object, you may add custom information in the form key: value, where key is a valid string. They are called object properties and can be approached in many different ways. The value can be anything legal, as simple as numbers or strings, and also functions, or even other objects. Three simple ways: in the declaration, dot notation and square brackets

var markerA = new google.maps.Marker({
    map: map,
    position: new google.maps.LatLng(0, 0),
    customInfo: "Marker A"
});

var markerB = new google.maps.Marker({
    map: map,
    position: new google.maps.LatLng(-10, 0)
});
markerB.customInfo = "Marker B";

var markerC = new google.maps.Marker({
    map: map,
    position: new google.maps.LatLng(-20, 0)
});
markerC['customInfo'] = "Marker C";

Then to retrieve it in a similar manner:

google.maps.event.addListener(markerA, 'click', function() {
    alert(this.customInfo);
});