Writing middleware functions for Mongoose

Middleware functions in Mongoose are also called hooks. There are two types of hooks pre hooks and post hooks.

The difference, between pre hooks and post hooks, is pretty simple. pre hooks are called before a method is called, and post hooks are called after. For example:

      const UserSchema = new Schema({ 
          firstName: String, 
          lastName: String, 
          fullName: String, 
      }) 
      UserSchema.pre('save', async function preSave() { 
          this.fullName = `${this.lastName} ${this.firstName}` 
      }) 
      UserSchema.post('save', async function postSave(doc) { 
          console.log(`New user created: ${doc.fullName}`) 
      }) 
      const User = mongoose.model('User', UserSchema) 

And later on, once the connection is made to the database, within an async function:

      const user = new User({ 
          firstName: 'John', 
          lastName: 'Smith', 
      }) 
      await user.save() 

Once the save method is called...