Question

I’m currently using the following code the place multiple markers on a Google Map using their API.

The problem I’m having is with multiple infowindows not working (only showing the last one).

There are loads of questions like mine here on SO. Actually make that a shitload of questions 🙂

Just an example: Trying to bind multiple InfoWindows to multiple Markers on a Google Map and failing

The solution to my problem is kinda easy: just enclose the click listener to a (anonymous) function.

However what I don’t understand is why my solution isn’t working (saving the markers and infowindows in arrays instead of just one variable).

    var markers = [];
    var infowindows = [];

    // add shops or malls
    for (var key in data.markers) {
      if (data.markers.hasOwnProperty(key)) {
        infowindows[key] = new google.maps.InfoWindow({
            content: data.markers[key].infowindow
        });

        markers[key] = new google.maps.Marker({
                position: new google.maps.LatLng(data.markers[key].location.lat, data.markers[key].location.lng),
                map: map,
                flat: true,
                title: data.markers[key].name,
                draggable: false
        });
        var iconFile = 'http://maps.google.com/mapfiles/ms/icons/'+marker_color+'-dot.png';
        markers[key].setIcon(iconFile);

        google.maps.event.addListener(markers[key], 'click', function() {
          infowindows[key].open(map, markers[key]);
        });
      }
    }

So… I don’t what to get the solution how to get it working with some function to enclose the listener (although it should work, haven’t tested it yet but will), but I want to know the reason why it wouldn’t work if I add the markers and infowindows to arrays instead.

Answer 1

Javascript has a language structure called “closures”. Closures are functions (such as the function() {} you declare above to deal with click listener) which capture references to external variables.

There are plenty of resources which explain them better than I can, which I suggest you consult, but here’s my best attempt:

In this block here:

    google.maps.event.addListener(markers[key], 'click', function() {
      infowindows[key].open(map, markers[key]);
    });

Because “key” is already defined as an external variable, the function will capture a reference to that variable. So where you expect:

infowindows["helloworld"]

Javascript will instead interpret this as:

infowindows[reference to key]

When you click on a marker, it looks up “reference to key” to see what the current value of key is. Because this probably won’t happen until your loop has finished, key will be equal to whatever the last key in your data.markers object is. And it will be equal to that value for EVERY click listener you added.

The solution, as you point out, is to wrap this in an anonymous function to get Javascript to evaluate the value of “key” at the time that the click listener is added.

  google.maps.event.addListener(markers[key], 'click', function(innerKey) {
      return function() {
          infowindows[innerKey].open(map, markers[innerKey]);
      }
    }(key));