Markers not displaying on google map, but no errors in console

If you have a collection of marker i think you should use this way iteranting on the collection and assignd the i element :

for(var i =0; i<locations.length; i++){  addMarker(locations[i].lat, locations[i].lng);}

proof of concept fiddle

You may give it a try by changing to below.

while ($row = mysqli_fetch_assoc($result)) {    //Assuming "lat" is column name in tester table. Please change it if required.    $lat= $rows['lat'];    //Assuming "lng" is column name in tester table. Please change it if required.    $lng= $rows['lng'];    $list[] = array('lat' => $lat, 'lng' => $lng);}

I made this using prototypes, the code is much more cleaner. An example (supose you are using jquery):

Create 3 scripts:

1. Map script
2. JSONParser script
3. Main script

First Create the prototype functions for work with google maps:

 var MapUtil = (function($) {    var mapProp = {        center: new google.maps.LatLng(-38.4529181, -63.5989206),        zoom: 4,        mapTypeId: google.maps.MapTypeId.ROADMAP    };    /**     * @returns     * Maps options for create a google map     */    function getMapProperties() {        return mapProp;    }    /**     * Create simple marker     * @param lat     * Latitude for the marker     * @param lng     * Longitude for the marker     * @param myMap     * Actual map     * @returns     * Return a new google map marker     */     function simpleMarker(lat, lng, myMap) {         var marker = new google.maps.Marker({             //add the marker options             position: new google.maps.LatLng(lat, lng),             draggable:true,             animation: google.maps.Animation.DROP,             map: myMap,         });        return marker;     }    return {        getMapProperties: getMapProperties,        simpleMarker: simpleMarker,    };})(jQuery);

Then create the JSONParser for transform the JSON to javascript objects:

var JSONParser = (function($){    /**     * @return An array with the markers parsed     */    function getMarkers(json) {        var markers = [];        for (int i = 0; i < json.length; i++) {            markers.push({json[i].lat,json[i].lng});        }        return markers;    }    return {        getMarkers : getMarkers,    };})(jQuery);

And you main script:

$(document).ready(function() {    var myMap = null;    //add the map to the page    google.maps.event.addDomListener(window, 'load', initialize);    /**     * Init Google Maps     */    function initialize() {        var map=new google.maps.Map(document.getElementById("googleMap"),                 MapUtil.getMapProperties());        myMap = map;        createMarkers();    }    function createMarkers() {        //retrieve the information        $.ajax({                         type: 'post',            url: 'MarkersController.php',            data: {},            success: function (data) {                var json= $.parseJSON(data);                var markers = JSONParser.getMarkers(json);                for (var i = 0; i < markers.length; i++) {                    MapUtil.simpleMarker(marker[i].lat, marker[i].lng, myMap);                }            }        });    }});

Then add the scripts to the html in this order:

<script src="...path.../map-utils.js"></script><script src="...path.../json-pareser.js"></script><script src="...path.../main.js"></script>

For other functionality about the map or json add extra methods!