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Getting a central point from an esri multipoint with javascript

I have created a webpage displaying markers on an ersi map using javasvipt.
Data:
MapNorth MapEast
439624 504743
439622 504736
439722 504775
439738 504739
439715 504774
439734 504739
The javascript code:
var points = data.map(function(x){
return [x.MapEast, x.MapNorth];
});
var myMultiPoint = {"geometry":{"points":points,"spatialReference":27700},"symbol":{"color":[255,255,255,64],
"size":6,"angle":0,"xoffset":0,"yoffset":0,"type":"esriSMS","style":"esriSMSCircle",
"outline":{"color":[0,0,0,255],"width":6,"type":"esriSLS","style":"esriSLSSolid"}}};
var gra = new esri.Graphic(myMultiPoint);
myMap.graphics.add(gra);
var graExtent = esri.graphicsExtent(myMap.graphics.graphics);
myMap.setExtent(graExtent);
What the above code does is plot markers on the map and then zooms into the extent. What my employers want now is for me to find the central point of all of those points and display one marker in the center.
Can this be done? If so and you tell me how?
Thanks
Paul

Couple of things.
Did you know about gis.stackexchange.com? They might better solve your problem.
What you're trying to do is find the centre of a polygon assuming those points aren't all in a line.
Here's a link with an answer to the question I think you're asking https://gis.stackexchange.com/questions/7998/how-can-i-calculate-the-center-point-inside-a-polygon-in-arcgis-9-3
The solution posted there uses getExtent().getCenter() as seen here
var myPolygonCenterLatLon = myPolygon.getExtent().getCenter();

I think what you want to be doing here is instead of creating a Multipoint, create a Polygon from your array of points. Once you have a polygon defined, you can do something like
var myPolygon = new Polygon(points);
var centroid = myPolygon.getCentroid();
This should get you the centroid of the points making up the Polygon.
https://developers.arcgis.com/javascript/jsapi/polygon-amd.html
Note that this requires at least version 3.7 of the JS API, though.

One thing to point out to those trying to using .getCentriod() , make sure your polygon is closed. Your 1st point and Last Point need to be in the same spot. Otherwise it wont work right. ( I ran into this a year ago, not sure if they changed this)

Distance between several latlng with Google maps API

If I have stored in my DB several latlng points, and I want to compare those points with my actual latlng position( giving me the distance between each latlng points with my actual latlng), how would that be posible with google maps API? or it's something that it would be easier using my database?

Computing the distance between two points on a sphere requires the use of the haversine formula, which requires a pretty solid understanding of trigonometry.
The easier way would be to leverage the Google Maps API which has the handy function computeDistanceBetween in the google.maps.geometry.spherical namespace.
Here's some sample code for using computeDistanceBetween:
var home = ['London', new google.maps.LatLng(51.5, -0.1167)];
var pts = [
['Prague', new google.maps.LatLng(50.08, 14.43)],
['Paris', new google.maps.LatLng(48.856614, 2.3522219000000177)],
['Berlin', new google.maps.LatLng(52.5200065999, 13.404953999999975)]
];
// provide a shortcut for the distance function
var dist = google.maps.geometry.spherical.computeDistanceBetween;
pts.forEach(function(pt){
var d = dist(home[1], pt[1])/1000;
// d is now the distance (in km) between the home coordinate and the point
});
See working example here: http://jsfiddle.net/aJTK2/5/

If you intend to use your database for this sort of work, you might want to think about using PostGIS for this. With PostGIS installed:
CREATE EXTENSION postgis;
SELECT ST_Distance(
ST_GeographyFromText('POINT(115.764286 -31.746416)'),
ST_GeographyFromText('POINT(151.036606 -33.906896)')
);
Produces:
st_distance
------------------
3295294.42439749
(1 row)
Compared to Google Maps output, which thinks it's about 3700 km (walking, not crow-flies).
So that seems about right, distance wise.
Note that this is spheroid distance, i.e over the earth's surface, not point-to-point through it.
Watch out for the co-ordinate order in PostGIS vs Google Maps.
To learn more about PostGIS:
Introduction to PostGIS
Introduction to PostGIS - Geography
PostGIS.net

ArcGIS JavaScript API (3.8) centerAndZoom does not appear to be working

I'm new to ESRI's JavaSCript API and am very impressed by its ease of use and speed. As part of a interactive data portal I have users enter latitude and longitude as decimal degrees as part of a spatial query to return State, County and FIPs. That part works just fine, but as an added feature I want to draw a dot graphic on an existing map showing the entered coordinates location (DONE) then Center and Zoom to said point at some reasonable scale.
The centerAndZoom method is the logical choice here, but it doesn't seem to be working. My sense is that the Map needs to be refreshed but I can't seem to figure this one out.
I'm sure I'm missing something fundamental here; Thanks in advance for your time!
function DrawPointAndZoom() {
// Get currently entered lat/long.
var lat = $('#SiteLatitude').attr('value');
var long = $('#SiteLongitude').attr('value');
var latLongPoint = new esri.geometry.Point(long, lat, new esri.SpatialReference({ wkid: 4326 }));
//Draw point
var symbol = new esri.symbol.SimpleMarkerSymbol().setSize(8).setColor(new dojo.Color([255, 0, 0]));
var graphic = new esri.Graphic(latLongPoint, symbol);
var infoTemplate1 = new esri.InfoTemplate();
infoTemplate1.setTitle("point1");
infoTemplate1.setContent("test point 1");
graphic.setInfoTemplate(infoTemplate1);
map.graphics.add(graphic);
map.centerAndZoom(latLongPoint, 15);
}

95% of the time, anything that doesn't work with a Point in the JS API is due to the spatialreference being wrong. :)
Check that your map's SR (map.spatialreference.wkid) is the same as the point's (4326, as you've defined it here.) You may need to use everyone's favourite function, geographicToWebMercator if the map is using one of its usual Web Mercator coordinate systems.
Edited with details from Tony's comment/answer:
var webMercPoint = esri.geometry.geographicToWebMercator(latLongPoint)
{Missing code here}
map.centerAndZoom(webMercPoint, 15);

Yes! thanks for taking the time to respond. I went down that hole yesterday and tried it with a webMercatorPoint... and viola it worked!
var webMercPoint = esri.geometry.geographicToWebMercator(latLongPoint)
{Missing code here}
map.centerAndZoom(webMercPoint, 15);
Much Appreciated!

LatLong falls within a given polygon in D3 + Leaflet

I am trying to learn how to use the Javascript library leaflet along with d3 to create various map visualisations.
I have been following this tutorial which creates a choropleth map of the United States with some interactivity. This provides some of what I need, but the main functionality I want is to have a list of lat/long coordinates classified according to which region they belong to.
This would mean, in the tutorial map for example, if I had a lat long value (55, -3) which fell within the state of Arizona's polygon, the program could classify this point as belonging to Arizona.
Is there a function in the leaflet (or d3) library which will allow me to enter a lat long coordinate as a parameter and return the name of the feature it belongs to? The tutorial above allows you to attach a function to every feature via the onEveryFeature property and can fire mouseover events when each feature is hovered over. Surely there is a way to extend this functionality to numerically entered data instead of mouse points?

Leaflet would need some tweaking if you wish to do this. It leaves the handling of mouseclicks to the browser and therefore does not need logic for determining if a point lies inside a polygon.
I am not very knowledgeable about d3 but it's not glaringly obvious to me how it'd do this out of the box. Looking at the polygon code, I do find a clipping algorithm and intersection of infinite lines.
If you add a third library, however, this should be rather simple.
The OpenLayers Geometry library can determine if a point lies inside a polygon.
EDIT: I got this to work, see also http://jsfiddle.net/VaY3E/4/
var parser = new OpenLayers.Format.GeoJSON();
var vectors = parser.read(statesData);
var lat = 36;
var lon = -96;
var point = new OpenLayers.Geometry.Point(lon, lat);
for( var i = 0; i< vectors.length; i++ ){
if(vectors[i].geometry.intersects(point)){
alert(vectors[i].attributes['name']);
}
}
Or you could use https://github.com/maxogden/geojson-js-utils , a bit more specific library. It looks like it knows how to read GeoJSON and it has a method gju.pointInPolygon. I've not tested it though.

custom google maps icons

Is there any way to create custom dynamic icons for the markers?
What I want to create is a list of points and put a number "n" within markers specifying it's the n-th element.

Yes. You can either use custom icons with Numeric Markers or the built-in numbering system on Google has for markers.
Assuming the last option (not using custom markers), your marker definition code would look like:
var NUMBER_SHOWN='34';
var ICON_COLOR = 'ff6600';
var ICON_TEXT_COLOR = '000000';
var marker = new google.maps.Marker({
position: [LatLong Obj],
map: [Map Obj],
icon: 'http://chart.apis.google.com/chart?chst=d_map_pin_letter&chld='+NUMBER_SHOWN+'|'+ICON_COLOR+'|'+ICON_TEXT_COLOR
});
I believe this only works for two-digit number, though I'm not sure.
EDIT
Geocodezip has correctly pointed out that the second approach has now been deprecated, so it looks like you're stuck using the first approach (using google's custom numbered-icons). Have a look at Daniel Vassallo's answer here on how to use it. If none of the markers fit your needs (colors, look, etc) you can create your own custom markers for each of the numbers (if you know how, you can write a server-side script that you can pass GET vars to and have it build the icon for you using GD etc, but you can also just build all the icons by hand)