how i learn typescript

learn-typescript.ts

type Tables = {
    person: {
        id: number;
        first_name: string;
    },
    product: {
        id: string;
        name: string;
        createdAt: Date
    }
};

type TableNames = keyof Tables;

type Alias<T extends string> = T extends `${string} as ${infer A}` ? A : never;
type Original<T extends string> = T extends `${infer O} as ${string}` ? O : never;

type SqlAlias<ColumnNames extends string> = ColumnNames extends ColumnNames ?
    `${ColumnNames} as ${string}` :
    never;

type Columns<ColumnNames extends string> = ColumnNames | SqlAlias<ColumnNames>;

type SelectReturn<
    Name extends TableNames,
    Column extends Columns<keyof Tables[Name] & string>
> = {
    [C in Column as 
        Alias<C> extends never ? 
        C : 
        Alias<C>
    ]: C extends keyof Tables[Name] ? 
        Tables[Name][C] : 
            Original<C> extends keyof Tables[Name] ?
            Tables[Name][Original<C>] :
            never; // unreachable
}

declare function select<
    Name extends TableNames,
    Column extends Columns<keyof Tables[Name] & string>
>(tableName: Name, columns: Column[]): 
    SelectReturn<Name, Column>;

const person = select('person', ["first_name as firstName", "id"])
const product = select('product', ['createdAt', 'name as productName'])

type P1 = Flat<typeof person>;
type P2 = Flat<typeof product>;

type Flat<T> = {
    [K in keyof T]: T[K];
}

Awesome, though I got only 70% of the codebase. 😅