How to create a document only if it doesn't exist already using Mongoose?

You can use Model.findOneAndModify():

const dancingTopic = await Topic.findOneAndModify(
  { name: 'Dancing' },
  {},
  { upsert: true, new: true }
)

This will either return the existing topic if it already exists or create a new topic with name: 'Dancing' if it doesn't and return it.

The second argument, {}, is the update object with the fields to update for the existing/new document. Here we just pass an ampty object, but if there is a field you want to update, you can specify it there.

The upsert: true option makes sure that a new document is created if one doesn't exist.

The new: true option makes sure that you get the newly created object or the updated object. If this is not set, you get null if the object didn't exist and was created, or the version before the update was applied if the object existed and was updated.