[MongoDB] add dependencies and global configuration

package.json

@@ -9,6 +9,7 @@
     "test": "jest"
   },
   "dependencies": {
+    "mongodb": "^3.6.5",
     "next": "10.0.9",
     "react": "17.0.1",
     "react-dom": "17.0.1"

util/mongodb.js

@@ -0,0 +1,48 @@
+import { MongoClient } from "mongodb";
+
+const { MONGODB_URI, MONGODB_DB } = process.env;
+
+if (!MONGODB_URI) {
+  throw new Error(
+    "Please define the MONGODB_URI environment variable inside .env.local"
+  );
+}
+
+if (!MONGODB_DB) {
+  throw new Error(
+    "Please define the MONGODB_DB environment variable inside .env.local"
+  );
+}
+
+/**
+ * Global is used here to maintain a cached connection across hot reloads
+ * in development. This prevents connections growing exponentially
+ * during API Route usage.
+ */
+let cached = global.mongo;
+
+if (!cached) {
+  cached = global.mongo = { conn: null, promise: null };
+}
+
+export async function connectToDatabase() {
+  if (cached.conn) {
+    return cached.conn;
+  }
+
+  if (!cached.promise) {
+    const opts = {
+      useNewUrlParser: true,
+      useUnifiedTopology: true,
+    };
+
+    cached.promise = MongoClient.connect(MONGODB_URI, opts).then((client) => {
+      return {
+        client,
+        db: client.db(MONGODB_DB),
+      };
+    });
+  }
+  cached.conn = await cached.promise;
+  return cached.conn;
+}