I tried to connect to my db but it keep on returning that the collection is undefined even though I already declared it as a global variable. Not really sure what the closure issue is.

const express = require('express');

const app = express(); 

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');
let db;
MongoClient.connect('mongodb://localhost:27017/todos', { useUnifiedTopology: true },  (err, client) => {
  assert.equal(nul, err);

  db = client.db('todos'); 
  db.collection('todos').insertMany([
    { done:true, desc: 'write code'},
    { done:true, desc: 'write code'},
    { done:true, desc: 'write code'},
  ]);
  client.close();
})


app.get('/', (req, res) => {
    res.json('did this work!');
}); 


app.get('/todos', async (req, res) => {
    const todos = await db.collection('todos').find().toArray();
    res.json(todos);
  });
  
app.listen(3001, () =>{
    console.log("work pls");
});