Hello @Daniele_Poletti, welcome to the community forums!
Without having much context on the rest of your application, I see one thing in the code provided that might be confusing your app: the JSON parse function.
Have you tried simplifying your code to:
const db = client.db(‘expManger’);
const collection = db.collection(‘cdc’);
const doc = {"description": "prova testo fisso"};
const result = await collection.insertOne(doc);
console.log(`New document with _id: ${result.insertedId}`)