The purpose this code is to study about more complex type system of typescript. Its a query builder type system

example.ts

interface User {
  id: number;
  name: string;
  email: string;
  password: string;
  updated_at: Date;
}

interface Product {
  id: number;
  name: string;
  description: string;
  price: number;
}

interface Order {
  id: number;
  user_id: number;
  product_id: number;
  quantity: number;
  created_at: Date;
}

type DB = {
  user: User;
  product: Product;
  order: Order;
}

const myDbInstance = {} as DB;

const getUserById = async (id: number) => {
  const user = await QueryBuilder.database(myDbInstance)
    .from("user")
    .select(["id", "email", "name"])
    .where("id = :id", { id })
    .build();
  
  return user.name;
}

query-builder.ts

type QueryBuilderMethods<
  DB,
  Table extends keyof DB = keyof DB,
  Columns extends keyof DB[Table] = keyof DB[Table],
  ColumnsSelected extends Array<Columns> = Array<Columns>,
> = {
  from: <T extends Table>(table: T) => QueryBuilderMethods<DB, T>,
  select: <S extends Array<Columns> = Array<Columns>>(s: S) => QueryBuilderMethods<DB, Table, Columns, S>,
  where: <
    W extends MakeWhereString<Columns & string>,
    V extends ExtractColumnByWhere<W> 
  >(
    w: W,
    v: { [K in V]: DB[Table][V & Columns] }
  ) => QueryBuilderMethods<DB, Table, Columns, ColumnsSelected>,
  build: () => Promise<Pick<DB[Table], ColumnsSelected[number]>>
}

type MakeWhereString<
  Column extends string, 
  Operator extends string = ComparisonOperators, 
  Value extends string = string
> = `${Column} ${Operator} :${Value}`;

type ComparisonOperators = "=" | "<>" | ">=" | "<=";

type ExtractColumnByWhere<Str> = Str extends MakeWhereString<infer C>
  ? C
  : never;

declare namespace QueryBuilder {
  function database<DB>(db: DB): QueryBuilderMethods<DB>;
}

interface User {
  id: number;
  name: string;
  email: string;
  password: string;
  updated_at: Date;
}

interface Product {
  id: number;
  name: string;
  description: string;
  price: number;
}

interface Order {
  id: number;
  user_id: number;
  product_id: number;
  quantity: number;
  created_at: Date;
}

type DB = {
  user: User;
  product: Product;
  order: Order;
}

const myDbInstance = {} as DB;

const getUserById = async (id: number) => {
  const user = await QueryBuilder.database(myDbInstance)
    .from("user")
    .select(["id", "email", "name"])
    .where("id = :id", { id })
    .build();
  
  return user.name;
}

The inspiration to build this code -> How i learn typecript

Some query builder references:

• Kysely
• Typeorm
• Prisma