Use blocked scoped variables

In addition to using const and let, this also upgrades our linter config and removes lint (mostly whitespace).

examples/layer-extent.js

@@ -8,21 +8,21 @@ function transform(extent) {
   return transformExtent(extent, 'EPSG:4326', 'EPSG:3857');
 }
 
-var extents = {
+const extents = {
   India: transform([68.17665, 7.96553, 97.40256, 35.49401]),
   Argentina: transform([-73.41544, -55.25, -53.62835, -21.83231]),
   Nigeria: transform([2.6917, 4.24059, 14.57718, 13.86592]),
   Sweden: transform([11.02737, 55.36174, 23.90338, 69.10625])
 };
 
-var base = new TileLayer({
+const base = new TileLayer({
   source: new TileJSON({
     url: 'https://api.tiles.mapbox.com/v3/mapbox.world-light.json?secure',
     crossOrigin: 'anonymous'
   })
 });
 
-var overlay = new TileLayer({
+const overlay = new TileLayer({
   extent: extents.India,
   source: new TileJSON({
     url: 'https://api.tiles.mapbox.com/v3/mapbox.world-black.json?secure',
@@ -30,7 +30,7 @@ var overlay = new TileLayer({
   })
 });
 
-var map = new Map({
+const map = new Map({
   layers: [base, overlay],
   target: 'map',
   view: new View({
@@ -39,7 +39,7 @@ var map = new Map({
   })
 });
 
-for (var key in extents) {
+for (const key in extents) {
   document.getElementById(key).onclick = function(event) {
     overlay.setExtent(extents[event.target.id]);
   };