Google Maps API v3 : Passing values to Listener function(), so as to make a circle when the marker is clicked?

Question

I want to form a circle around the marker when the marker is clicked!

So, Listener is added to marker and the function needs to act on the circle.

Here is my Code :

   for(var j=0;j<lat.length;j++)
            {
                var pos = new google.maps.LatLng(lat[j],lng[j]);
                var marker_options = {position: pos, map:map, draggable:false};
                marker[j] = new google.maps.Marker(marker_options);
                circle_option = {strokeColor:"#FF0000",strokeOpacity:0.8,strokeWeight:2,fillColor:"#FF0000",fillOpacity:0.35,map:null,center:pos,radius:500};
                circle[j] = new google.maps.Circle(circle_option);
                google.maps.event.addListener(marker[j], 'click', function() {circle[j].setMap(map);});  // Error : circle[j] is undefined
            }

Error: circle[j] is undefined ?? (On the event.addListener line !)

Why.. it should be defined there ?

How to do this is the right way ? Please help!

Answer 1

You have a closure problem with j. When your function is called, j will reference the last value that j had in the for loop. So, j will be lat.length which is larger than the size of circle. The solution is to force j to be evaluated when generating the callback function:

function make_callback(circle, map) {
    return function() {
        circle.setMap(map);
    };
}

and then, in your loop:

google.maps.event.addListener(marker[j], 'click', make_callback(circle[j], map));

Wrapping the callback generation in a separate function will give you the value of circle[j] at the instant when you call make_callback rather than the value when the callback is called.

j is a reference to a value, the value that it points at depends on when you ask j what its value is. When you bind a function like this:

google.maps.event.addListener(marker[j], 'click', function() { something(j); });

The anonymous function doesn't ask j what its value is until the function is called, the function simply remembers that it is going to use j. When the callback function is executing, it will ask j for its current value and use that. This means two things:

• All the callbacks that you bound in that loop will use the same value of j.
• j will be lat.length as that's the last value that j was assigned during the loop.

By using the make_callback function to build the callbacks, we're asking j for its value at the time that we're binding the callback. This is just a standard trick to force j to be evaluated when it has the value we want.