Why does an Esri Extent load for one map layer but not for another? ArcGIS Javascript API

I am having the weirdest problem right now using the ArcGIS Javascript API (v2.4). I'm merely trying to create an instance of an ESRI topo map with an extent, and then add a layer.

Here is the code that works. I create an extent, then a map, then a streetmap layer and then finally add that layer.

var startExtent = new esri.geometry.Extent(-71.5, 42, -71, 42.5, new esri.SpatialReference({wkid:4326}) );
map = new esri.Map("map_canvas", { exte开发者_开发技巧nt: startExtent,fitExtent:false });
var streetmap = new esri.layers.ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer");
map.addLayer(streetmap);

However when I try and do the same thing with a separate server, it doesn't work.

var startExtent = new esri.geometry.Extent(-71.5, 42, -71, 42.5, new esri.SpatialReference({wkid:4326}) );
map = new esri.Map("map_canvas", { extent: startExtent,fitExtent:false });
var basemap = new esri.layers.ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer");
map.addLayer(basemap);

I know that the faulty-layer's server works, because if I create a map with no extent, it shows the full world imagery server, so it appears that creating a map in with one layer and an extent works, while creating a map with a different layer but the same extent does not work.

Any ideas why?

In your second example, the layer is in web Mercator.

Try converting the extent from geographic to web Mercator before using it in the map constructor.

The easiest way to convert the extent is to use esri..geometry.geographicToWebMercator.