Example of using bcrypt with mongoose middleware to enforce password hashing with bcrypt on save.

user.js

var mongoose = require('mongoose'),                                                                                                                                            
    Schema = mongoose.Schema                                                                                                                                                                                                                                                                                                                                
                                                                                                                                                                               
var bcrypt = require('bcrypt')                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                              
var UserSchema = new Schema( {                                                                                                                                                 
    email: String,                                                                                                                                                             
    password: String                                                                                                                                                           
} )                                                                                                                                                                                                                                                                                                                                                          
                                                                                                                                                                               
// pre                                                                                                                                                                         
UserSchema.pre('save', function(next) {                                                                                                                                        
    if(this.password) {                                                                                                                                                        
        var salt = bcrypt.genSaltSync(10)                                                                                                                                     
        this.password  = bcrypt.hashSync(this.password, salt)                                                                                                                
    }                                                                                                                                                                          
    next()                                                                                                                                                                     
})                                                                                                                                                                                                                                                                                                                                                          
                                                                                                                                                                               
mongoose.model('User', UserSchema);

For anybody who might need this for readability and clarity, better to use "isModified" so that when the password is updated, we don't have to re-hash again multiple times.

userSchema.pre("save", async function(next){
if(this.isModified("password")){
this.password = await bcrypt.hash(this.password, 8)
}
next();
});