How do I get the legend from a ArcGISDynamicMapServiceLayer using esri javascript api?

Question

New Code: I updated the code a bit to more a recent recent version of the api, but ashima was correct in that I was not passing my layer as an array. Fixed that and it worked

 map = new Map("map", {                     
   basemap: "streets",
   center: [ -84.32, 46.5], // long, lat
    zoom: 13
});             

//fetches myMap layer
var myMap = new FeatureLayer(myMapLocation, {
  mode: FeatureLayer.MODE_ONDEMAND,                     
  outFields:["*"]
});

//add the legend
map.on("layers-add-result", function (evt) {                    
    var layerInfo = arrayUtils.map(evt.layers, function (layer, index) {
    return {layer:layer.layer, title:layer.layer.name};
});                         
if (layerInfo.length > 0) {                 
    var legendDijit = new Legend({
        map: map,
        layerInfos: layerInfo
    }, "legend");
legendDijit.startup();
    }
});

map.addLayers([myMap]);

So I've been using the ESRI Javascript API, so far what I've been able to do is create a map, fetch a layer from another source and combine them, but I'm stuck on grabbing the legend from the other source and also adding that. This is what I currently have after my attempts (also tried switching the map : map to map : mymap) The myMapLocation is just set to "/MapServer" and not "/mapServer/layers" or anything

 map = new Map("mapDiv", {
                    basemap: "streets"                      
                });
myMap = new esri.layers.ArcGISDynamicMapServiceLayer(myMapLocation);

map.on('layers-add-result', function () {
//this section breaks v              
    var legend = new Legend({
        map: map,
        layerInfos: legendLayers
    }, "legendDiv");
// ^
    legend.startup();
});

legendLayers.push({ layer: myMap });

map.addLayers([myMap]);

Answer 1

It looks like you have not declared legendLayers as an array (var legendLayers = [];) before using it. If that doesn't solve your problem, could you paste the error that you get when you run this code in the browser?