Even with SQL databases you have to deal with the fact that the web app UI is showing the item as available to multiple users, but it should let only the first one book it. This is not special to the MERN stack.
The simplest approach is to have a collection of bookable items, with a booked field. When booking an item you'll use an update() to set booked to true. In the update, in addition to the item ID in the filter, you'll add a booked = false criterion. If it fails (saying such a record doesn't exist), it means someone else has grabbed it, and show a message to the user.
For a more generic solution where you want to prevent overwrites, many people use a version field which is updated every time the document is updated. For MongoDB, an ObjectID is a natural choice for the data type of this field. When updating a document, the caller has to supply the value of the version field from the find() call, and handle a failure as a conflict.
https module. You'll find tons of tutorials on the net on how to do this.dangerouslySetInnerHTML()).