Refactoring non-JS like code

src/app/hooks/selectFromDB.js

@@ -27,18 +27,18 @@ var displacement = 0;
 
 var ID = 0;
 
-var fetched_data = new Map([
-  ["pluv", new Set()],
-  ["flood", new Set()],
-  ["rain", new Set()],
-  ["river", new Set()],
-]);
+var fetched_data = {
+  pluv: new Set(),
+  flood: new Set(),
+  rain: new Set(),
+  river: new Set(),
+};
 
 function is_valid(id, category) {
-  if (fetched_data.get(category).has(id)) {
+  if (fetched_data[category].has(id)) {
     return false;
   } else {
-    fetched_data.get(category).add(id);
+    fetched_data[category].add(id);
   }
   return true;
 }