Is testing (like mocha) necessary when writing a REST API?

Question

Software testing

written api for a web application, there are basically CRUD operations in the database. All errors are processed. Does it need to be tested with special frameworks at all? How to test something like this piece of code
`User.findById(id, (err, user) => {
if (err) throw err;
if (user === null) res.send('user not found');
// do something with the user
})`
Explain. thanks in advance