Use eslint and add its config.

.eslintrc

+{
+  "extends": "standard"
+}

package.json

@@ -13,5 +13,13 @@
     "body-parser": "^1.18.2",
     "express": "^4.16.2",
     "mongoose": "^4.13.2"
+  },
+  "devDependencies": {
+    "eslint": "^4.11.0",
+    "eslint-config-standard": "^10.2.1",
+    "eslint-plugin-import": "^2.8.0",
+    "eslint-plugin-node": "^5.2.1",
+    "eslint-plugin-promise": "^3.6.0",
+    "eslint-plugin-standard": "^3.0.1"
   }
 }