Password Hashing
password hashing is the process of transforming plaintext data into an unreadable* format through hashing techniques. 
plaintext refers to information that is fed as an input to a coding process
plaintext data, when stored as is, is often referred to as cleartext.
cleartext is what we want to avoid as this leads to a vulnerability and confidentiality when storing user credentials into a database. 
Hashing is the process of transforming a string of characters into another value. 
Hashing functions are one-way, meaning you cannot convert a hashed string back to its plaintext form.
*in human standards
ISO/IEC7498-2, Information Processing Systems--Open Systems Interconnection Reference Model--Part 2: Security Architecture

Password Hashing through Mongoose
Mongoose provides functionalities that make it easier to perform password hashing. 
Do note that it is still possible with the native mongodb driver, but additional code must be written first.
The ff steps are usually involved when password hashing:
User registers a user -> server checks if the username already exists
If not, server reads credentials in plaintext
Before the document is saved, the password must be hashed using some hashing algorithm.
The document with the hashed password is then saved into the database.
When logging in, the server hashes the input password and compares whether the hashed password from the database matches.

Password Hashing through Mongoose
To implement password hashing through mongoose, we will be using the following concepts:
mongoose middlewares – allows us to perform functions before a certain database operation is performed.
bcrypt – a npm module which makes use of the bcrypt hashing function 

mongoose middlewares
Mongoose also features middleware functions (aka pre and post hooks).
It allows you define working code for pre- and post- mongoose operation execution.
General Syntax for pre hooks:
schema.pre([operation], function (next) {...});
where:
operation – string representing the method name of the mongoose operation (e.g., find, save, insertMany, etc.)
function(next) – the working code to be executed. Calling next() will invoke the next middleware function for the same operation. 
https://mongoosejs.com/docs/middleware.html

Using a middleware for hashing
The following middleware function will be performed before the 'save' operation is executed. 
For save, this refers to the document being saved. Depending on the operation, this could be linked to other references. 
user.isModified('password') checks whether the password field of the document has been modified or is newly created.