Hi Wenhua,
I am running into the same problem as you are. I did the same as the video tutorial and it did not work, but the following lines worked for me.
import Map = require("esri/Map");
import FeatureLayer = require("esri/layers/FeatureLayer")
Maybe because the ArcGIS JS library we are getting from npm has been updated to version 4.2, not 3.18, which may have packaged classes and objects differently?
Additionally, though these two import lines worked, I am not getting IntelliSense creating the map constant, e.g., basemap, center. Any idea?