谷歌地图 API 向已经启动的地图添加标记

    var watchID;
    var geo;    // for the geolocation object
    var map;    // for the google map object
    var mapMarker;  // the google map marker object
    var markers;

    // position options
    var MAXIMUM_AGE = 200; // miliseconds
    var TIMEOUT = 300000;
    var HIGHACCURACY = true;
    function getGeoLocation() {
        try {
            if( !! navigator.geolocation ) return navigator.geolocation;
            else return undefined;
        } catch(e) {
            return undefined;
        }
    }

    function show_map(position) {
        var lat = position.coords.latitude;
        var lon = position.coords.longitude;
        var latlng = new google.maps.LatLng(lat, lon);
        ajax_post(lat,lon);
        if(map) {
            map.panTo(latlng);
            mapMarker.setPosition(latlng);
        } else {
            var myOptions = {
                zoom: 18,
                center: latlng,
                // mapTypeID --
                // ROADMAP displays the default road map view
                // SATELLITE displays Google Earth satellite images
                // HYBRID displays a mixture of normal and satellite views
                // TERRAIN displays a physical map based on terrain information.
                mapTypeId: google.maps.MapTypeId.ROADMAP
            };
            map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
            map.setTilt(0); // turns off the annoying default 45-deg view
            mapMarker = new google.maps.Marker({
                position: latlng,
                title:"You are here."
            });
            mapMarker.setMap(map);
            alert ('in elses statement after map');
           }
    }
    function geo_error(error) {
        stopWatching();
        switch(error.code) {
            case error.TIMEOUT:
                alert('Geolocation Timeout');
                break;
            case error.POSITION_UNAVAILABLE:
                alert('Geolocation Position unavailable');
                break;
            case error.PERMISSION_DENIED:
                alert('Geolocation Permission denied');
                break;
            default:
                alert('Geolocation returned an unknown error code: ' + error.code);
        }
    }
    function stopWatching() {
        if(watchID) geo.clearWatch(watchID);
        watchID = null;
        alert ('Hey You Said Stop Watching');
    }
    function startWatching() {
        watchID = geo.watchPosition(show_map, geo_error, {
            enableHighAccuracy: HIGHACCURACY,
            maximumAge: MAXIMUM_AGE,
            timeout: TIMEOUT
        });
    }
     function startmap() {
        if((geo = getGeoLocation())) {
            startWatching();
        } else {
            alert('Geolocation not supported.')
        }
    }

    function ajax_post(lat,lon){
    // Create our XMLHttpRequest object
    var hr = new XMLHttpRequest();
    // Create some variables we need to send to our PHP file
    var url = "send/mypostion.php";
    var vars = "postlat="+lat+"&postlon="+lon;
    hr.open("POST", url, true);
    // Set content type header information for sending url encoded variables in the request
    hr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    // Access the onreadystatechange event for the XMLHttpRequest object
    hr.onreadystatechange = function() {
        if(hr.readyState == 4 && hr.status == 200) {
            // a string looking like: "32.6986, -117.101" (from a comment)
            var return_data = hr.responseText;
            // alert(return_data);
        }
    }
    // Send the data to PHP now... and wait for response to update the status div
    hr.send(vars); // Actually execute the request
    }

所以我有这个函数 get_userlat（），当它被调用时，它应该去使用 ajax 从服务器获取用户 lattiude，但是当我调用这个函数时，它会在调用时执行任何操作。我正在尝试创建一个跟踪代码，以便您在输入他们的 PIN 和用户电子邮件时能够跟踪您的朋友。我的数据库有用户 LAT 和 LNG。

    function get_userlat(){
    // Create our XMLHttpRequest object
    var hr = new XMLHttpRequest();
    // Create some variables we need to send to our PHP file
    var url = "send/getfriendspostion.php";
    var vars = "postemail="+prompt("What is your friends email")+"&postpin="+prompt("What is your friends pin number");
    hr.open("POST", url, true);
    // Set content type header information for sending url encoded variables in the request
    hr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    // Access the onreadystatechange event for the XMLHttpRequest object
    hr.onreadystatechange = function() {
        if(hr.readyState == 4 && hr.status == 200) {
            var return_data = hr.responseText;
            //addMarker(return_data);
            addMarker(return_data);
            return return_data;
        }
    }
    // Send the data to PHP now... and wait for response to update the status div
    hr.send(vars); // Actually execute the request
    }
    function addMarker(location) {
    var markers=new google.maps.Marker({
          position:location,
          });
        markers.setMap(map);
    }