fix flow, add it to the build script

scripts/build.sh

@@ -4,11 +4,15 @@ set -eu
 
 ( cd "$(dirname "$0")/../"
 
-  vdir=src/css/vendors
+  ( cd src/css/vendors
 
-  wget "https://fonts.googleapis.com/css?family=Press+Start+2P" -O $vdir/ps2p.css
-  wget "https://unpkg.com/nes.css/css/nes.min.css" -O $vdir/nes.min.css
-  wget "https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css" -O $vdir/bootstrap.min.css
+    wget "https://fonts.googleapis.com/css?family=Press+Start+2P" -O ps2p.css
+    wget "https://unpkg.com/nes.css/css/nes.min.css" -O nes.min.css
+    wget "https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css" -O bootstrap.min.css )
+
+  ( cd src/js
+
+    flow main.js )
 
   echo "Done !"
 )

src/js/main.js

@@ -22,6 +22,11 @@ const snd = document.getElementById("audioplayer");
 if (!snd) {
   throw "invalid audio player";
 }
+if (!(snd instanceof HTMLAudioElement)) {
+  throw "invalid audio player";
+}
+
+snd.volume = 0.3;
 
 // get the json file `file_name` and call `callback` with the result
 const get_json_callback = function(file_name, callback) {
@@ -125,8 +130,6 @@ const new_game = function (multi) {
   get_json_callback("lvl_01.json", game);
 }
 
-snd.volume = 0.3;
-
 const snd_start = function() {
   snd.currentTime = 0.0;
   snd.play();