Add generics to MariaDB Node Connector

When working with an instance of a MariaDB database I noticed that the npm package used to connect did not support Typescript generics. This was something I felt I could fix, so I sent in a PR.

Why generics?

Generics allow us to replace loosely typed APIs, anywhere any is used in Typescript, with contextual type information. You can think of a generic as a wildcard type. These are useful in the context of connecting to a database as they allow us to typehint the return type of queries, as well as the argument lists of prepared statements, for example. This means that we get compile time safety when working with queries and statements.

I've added a short video below to demonstrate the advantage of this. You'll notice that when the email value is changed to a number, an error is shown as it does not match the type supplied to the prepared statement.

Code example from video:

type UserInsertParams = [string, Buffer, Buffer<ArrayBufferLike>]
const userInsertSQL = 'INSERT INTO users (email, password_hash, salt) VALUES (?, ?, ?)'
const userInsert = await conn.prepare<UserInsertParams>(userInsertSQL) // UserInsertParams passed as values type

const salt = crypto.randomBytes(16);
return await userInsert.execute([ // will only accept a value of type UserInsertParams
  email,
  crypto.pbkdf2Sync(password, salt, 310000, 32, 'sha256'),
  salt
])

Implementing generics

You can see the source PR at mariadb-connector-nodejs/pull/334

This is actually something that is easy to implement. Even though the mariadb connector has an extensive codebase, I knew that I was looking for a .d.ts file that contained the type definitions. Usually this file is generated on the fly, however in this instance it seemed to be static. Using the LSP command "go to definition" is was able to find the Prepared interface inside a callback.d.ts file.

I notticed that the interface lacked a generic argument, as expected. So to fix I added the following:

interface Prepare {} // before

interface Prepare<V> {} // after

This allowed for any method within the interface to receive a generic also. See the execute method below. Note that exceute did use a generic for it's result, but not for it's argument list.

execute<T = any>(values: any, callback: (err: SqlError | null, result?: T, meta?: any) => void): void; // before

execute<T = any>(values: V, callback: (err: SqlError | null, result?: T, meta?: any) => void): void; // after 

The full list of changes can been seen on the pr, mariadb-connector-nodejs/pull/334.

I'm always glad to be able to contribute to an open source library. Especially a contribution that benefits my use of said library.