add env var checking

.env.example

@@ -1,2 +1,4 @@
-DATABASE_URL='<database_url>'
-SECRET_KEY='<secret_key>'
+DB_URL='<database_url>'
+JWT_SECRET='<secret_key>'
+JWT_EXPIRES_IN='1d'
+PORT=5000

src/index.ts

@@ -1,6 +1,25 @@
 import express, { urlencoded, json } from 'express';
 import cors from 'cors';
 import api from '@/api';
+import { config } from 'dotenv';
+
+config();
+
+const checkEnvVars = () => {
+  const requiredEnvVars = ['PORT', 'DB_URL', 'JWT_SECRET', 'JWT_EXPIRES_IN'];
+  let error: Boolean = false;
+  requiredEnvVars.forEach((envVar) => {
+    if (process.env[envVar] === undefined) {
+      error = true;
+      console.log(`${envVar} is undefined`);
+    }
+  });
+  if (error) {
+    process.exit(1);
+  }
+};
+
+checkEnvVars();
 
 const port = process.env.PORT || 5000;