For my final project I'm creating a google map web app - building from what we learned in Pset8.

I've got over 1,000 locations to add to my map - which looks pretty crowded, as you can see:

enter image description here

I'm trying to use google map's own markercluster library - see: https://developers.google.com/maps/documentation/javascript/marker-clustering?utm_source=welovemapsdevelopers&utm_campaign=mdr-screencast

I've downloaded google's markercluster.js file, and the .png files for the cluster icons, to my local directory.

However, I can't get it to work. When I add in the very simple code suggested by google's documentation:

// Add a marker clusterer to manage the markers.
    var markerCluster = new MarkerClusterer(map, markers,
        {imagePath:[ADD IMAGE PATH HERE]});

I get the following warning from the Cloud9IDE: "'markerCluster' is defined but never used" - and no clustering happens on my map.

I'd really appreciate any hints on how to fix this. Many thanks in advance for your help.

My code is as follows:


// Google Map
var map;

// markers for map
var markers = [];

// info window
var info = new google.maps.InfoWindow();

// execute when the DOM is fully loaded
$(function() {

    // styles for map
    // https://developers.google.com/maps/documentation/javascript/styling
    var styles = [

        // hide Google's labels
            featureType: "all",
            elementType: "labels",
            stylers: [
                {visibility: "on"}

        // hide roads
            featureType: "road",
            elementType: "geometry",
            stylers: [
                {visibility: "off"}


    // options for map
        var options = {
            center: {lat: 52.066289, lng: -1.34051652}, // Banbury, England - roughly in the middle of the country W-E, and a bit S
                                                    // Looked like a good place centre the map, just from a visual perspective!
            disableDefaultUI: true,
            mapTypeId: google.maps.MapTypeId.ROADMAP,
            maxZoom: 14,
            panControl: true,
            styles: styles,
            zoom: 6,
            zoomControl: true

    // get DOM node in which map will be instantiated
    var canvas = $("#map-canvas").get(0);

    // instantiate map
    map = new google.maps.Map(canvas, options);

    // configure UI once Google Map is idle (i.e., loaded)
    google.maps.event.addListenerOnce(map, "idle", configure);


 * Adds marker for place to map.
function addMarker(place)
    // create marker and add to globally-declared array called "markers"
    var LatLng = {lat: place.Lat, lng: place.Long};
    var name = place.Site_Name + ", " + place.County;
    var markerIcon = 'http://maps.google.com/mapfiles/ms/micons/blue-pushpin.png';
    var marker = new google.maps.Marker({
        position: LatLng,
        map: map,
        icon: markerIcon,
        title: name
    var markerCluster = new MarkerClusterer(map, markers, {imagePath: '/markers/m'}); //NOTE THIS IS WHERE I SEE THE WARNING "'markerCluster' is defined but never used"

//Bunch of other javascript to create an info window for each marker and populate it, what to do to update the map when it's dragged and so on.
//Note - addMarker() gets called when the map is configured and every time it is updated (i.e. zoomed, dragged etc.)


% extends "layout.html" %}

{% block title %}
    Solar Map
{% endblock %}

{% block main %}

    <!-- fill viewport -->
    <div class="container-fluid">

        <!-- https://developers.google.com/maps/documentation/javascript/tutorial -->
        <div id="map-canvas"></div>

        <!-- http://getbootstrap.com/css/#forms -->
        <form id="form">
            <div class="form-group">
                <label class="sr-only" for="q">City, County, Postal Code</label>
                <input class="form-control" id="q" placeholder="City, County, Postal Code" type="text"/>


{% endblock %}



    <!-- https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta -->
    <meta charset="utf-8"/>
    <meta content="initial-scale=1, width=device-width" name="viewport"/>

    <!-- documentation at http://getbootstrap.com/, alternative themes at https://www.bootstrapcdn.com/bootswatch/ -->
    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>

    <!-- app's own CSS -->
    <link href="{{ url_for('static', filename='styles.css') }}" rel="stylesheet"/>

    <!-- https://developers.google.com/maps/documentation/javascript/ -->
    <script src="https://maps.googleapis.com/maps/api/js?key={{ key | urlencode }}&v=3.exp"></script>

    <!-- https://developers.google.com/maps/documentation/javascript/marker-clustering -->
    <script src="/cluster/markerclusterer.js" type="text/javascript"></script>

    <!-- http://jquery.com/ -->
    <script src="https://code.jquery.com/jquery-latest.min.js"></script>

    <!-- http://getbootstrap.com/ -->
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>

    <!-- https://github.com/corejavascript/typeahead.js/blob/master/doc/jquery_typeahead.md -->
    <script src="https://cdn.rawgit.com/corejavascript/typeahead.js/master/dist/typeahead.jquery.js"></script>

    <!-- http://handlebarsjs.com/ -->
    <script src="https://cdnjs.cloudflare.com/ajax/libs/handlebars.js/4.0.5/handlebars.min.js"></script>

    <!-- http://stewartjpark.com/Flask-JSGlue/ -->
    {{ JSGlue.include() }}

    <!-- app's own JavaScript -->
    <script src="{{ url_for('static', filename='scripts.js') }}"></script>

    <title>CS50 Final Project: UK Solar Map: {% block title %}{% endblock %}</title>


    <div class="container">
        <nav class="navbar navbar-default">
            <div class="container-fluid">
                <div class="navbar-header">
                    <button aria-expanded="false" class="navbar-toggle collapsed" data-target="#navbar" data-toggle="collapse" type="button">
                        <span class="sr-only">Toggle navigation</span>
                        <span class="icon-bar"></span>
                        <span class="icon-bar"></span>
                        <span class="icon-bar"></span>
                    <a class="navbar-brand" href="{{ url_for('index') }}"><span class="blue">C</span><span class="red">S</span><span class="yellow">5</span><span class="green">0</span> <span class="red">Final Project</span></a>
                <div class="collapse navbar-collapse" id="navbar">
                    <ul class="nav navbar-nav">
                        <li><a href="{{ url_for('index') }}">Home</a></li>
                        <li><a href="{{ url_for('about') }}">About</a></li>
                        <li><a href="{{ url_for('methodology') }}">Methodology</a></li>
                        <li><a href="{{ url_for('links') }}">Links</a></li>

            {% block main %}{% endblock %}



  • Assuming addMarker is called in a place loop. Have you tried instantiating one MarkerClusterer after the loop is finished (instead of a new object every time a marker is added to the array)? Or, on second review, is markers.push(marker) outside the function? Commented Nov 11, 2017 at 20:54

1 Answer 1


Thanks to @DinoCoderSaurus for your help - basically the problem was that I was trying to instantiate a MarkerClusterer for each individual marker that was added to the array. The solution was to delete the line

markerCluster = new MarkerClusterer(map, markers, {imagePath: '/static/markers/m'});

from the addMarker() function; and instead have it run once only in a separate update() function, which is where:

  • my code loops through each of the places in turn and calls addMarker() for each (i.e. adds all my markers to the array)

  • then instantiate the markerClusterer on the whole array.

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