ArcGIS API for JavaScript - Featurelayer problem with CORS

Question

I created simple simple web site:

        require([
      "esri/map",
      "esri/config",
      "esri/dijit/InfoWindowLite",
      "esri/InfoTemplate",
      "dojo/dom-construct",
      "dojo/domReady!"
    ], function (
        Map,
        esriConfig,
        InfoWindowLite,
        InfoTemplate,
        domConstruct
       ) {
        esriConfig.defaults.io.proxyUrl = "http://111.111.111.111:6969/agsproxy/proxy.ashx"
        esriConfig.defaults.io.alwaysUseProxy = true;
        var customExtentAndSR = new esri.geometry.Extent(-437000, -1244500, -409000, -1239500, new esri.SpatialReference({ wkid: 5514 }));
        map = new Map("mapDiv", { logo: false, extent: customExtentAndSR, nav: false, slider: false, fitExtent: true, fadeOnZoom: true, showAttribution: false });
        var infoWindow = new InfoWindowLite(null, domConstruct.create("div", null, null, map.root));

infoWindow.startup();
 map.setInfoWindow(infoWindow);
 var template = new InfoTemplate();
var featureLayer = new esri.layers.FeatureLayer("http://111.111.111.111:6969/ags/rest/services/VSLP2014/jprl_plochy/MapServer/0", {
 mode: esri.layers.FeatureLayer.MODE_SNAPSHOT,
 infoTemplate: template,
 outFields: ["*"]
 });
 map.addLayer(featureLayer);
    });

When I use "MODE_SNAPSHOT" for my featurelayer, layer is displayed and is possible to display infotemplate. But when I run my web site again, It seems to be problem with CORS (No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:12345' is therefore not allowed access.). Once is everything OK, once not. When I run my web site 10 times, approximatelly 5 times is problem with CORS. When I use "MODE_ONDEMAND" it´s OK but when I zoom my layer, problem with CORS is displayed and zoomed layer is not displayed. When I used this sample code with the same layer but as dynamicservicelayer with labels, It seems to be OK. Could someone help me with this problem ? Is it problem with settings in proxy ?

Answer 1

When communicating to a service using the Esri Javascript API, a proxy page will only avoid CORS issues if the proxy page uses the same URL prefix as the original web site.

So, if your web site is http://localhost:12345/path/to/index.html, the request to: http://111.111.111.111:6969/agsproxy/proxy.ashx?http://111.111.111.111:6969/ags/rest/services/VSLP2014/jprl_plochy/MapServer/0 ...is still crossing an origin and will trigger the CORS preflight and checks.

If you can run the proxy page somewhere behind http://localhost:12345, then you should not see the CORS errors.

I'm not sure why you only see the problem about half the time.