Do yarn add dotenv

package.json

@@ -11,6 +11,7 @@
     "@firebase/app": "^0.1.2",
     "@firebase/messaging": "^0.1.3",
     "body-parser": "^1.18.2",
+    "dotenv": "^4.0.0",
     "express": "^4.16.2",
     "mongoose": "^4.13.2"
   },

yarn.lock

@@ -308,6 +308,10 @@ doctrine@^2.0.0:
     esutils "^2.0.2"
     isarray "^1.0.0"
 
+dotenv@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/dotenv/-/dotenv-4.0.0.tgz#864ef1379aced55ce6f95debecdce179f7a0cd1d"
+
 ee-first@1.1.1:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/ee-first/-/ee-first-1.1.1.tgz#590c61156b0ae2f4f0255732a158b266bc56b21d"