What is a unique index in MongoDB and how do you create one?

A unique index forbids duplicate values for a field and speeds up lookups; create one with createIndex({ field: 1 }, { unique: true }).

await votes.createIndex(
  { 'email': 1 },
  { unique: true }
);

Benefits:

• Fast lookups - queries on indexed fields are faster
• No duplicates - inserting a duplicate value throws an error (code 11000)

Use unique indexes for fields like email addresses, usernames, or IDs.

Go deeper:

• Unique Indexes (MongoDB Manual) — single-field vs compound unique indexes, the duplicate-key error, and the one-null-only rule.