var map, baseLyr;
require(["esri/map", "esri/layers/ArcGISTiledMapServiceLayer", "dojo/domReady!"], function(Map,ArcGISTiledMapServiceLayer) {
// create map
map = new Map("mapDiv", {
center: [-117.719, 34.100],
zoom: 14
});
// add basemap with 50% opacity
baseLyr = new ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer", {opacity:0.5});
map.addLayer(baseLyr);
});var lyrOpac = map.getLayer(map.getLayer[0]); //assuming your the layer is at 0 index. lyrOpac.setOpacity(0.5);