Cleanup and simplify JS code

Change-Id: I393e22763c1a1d18878026bf459b747b150429fb
diff --git a/dev/js/src/palette.js b/dev/js/src/palette.js
index 8c9f3fa..c5c91ee 100644
--- a/dev/js/src/palette.js
+++ b/dev/js/src/palette.js
@@ -3,7 +3,7 @@
    * Created with http://tools.medialab.sciences-po.fr/iwanthue/
    * and H: 0-360, C: 0.4-2.1, L: 0.7-1.5, 64 colors
    */
-  var palette = [
+  const palette = [
     '9C8AC7',
     '6DED31',
     'DF7F23',