TypeScript method in parent class access Type in subclass

Question

I have one class:

export class DataAccess {
  tableName: string;
  constructor({ tableName }: { tableName: string }) {
    this.tableName = tableName;
  }
  async getWhere(where: any, { single = false } = {}) {
    const options: { limit?: number } = {};
    if (single) {
      options.limit = 1;
    }
    const results = await someDBobj[this.tableName].find(where, options);
    if (!single) {
      return results;
    }
    return results.length ? results[0] : null;
  }
}

And a subclass:

import { DataAccess } from "./data-access";

type UsersTable = {
  id: string;
  email: string;
  password: string;
  createdAt: Date;
};

export default class Users extends DataAccess {
  columns: { [P in keyof UsersTable]: string };
  constructor() {
    super({
      tableName: "users"
    });
    this.columns = {
      id: "id",
      email: "email",
      password: "password",
      createdAt: "createdAt"
    };
  }
}

And using the Users class:

await users.getWhere(
  {
    email: user,
    password
  },
  {
    single: true
  }
);

How can I set the Type of where: any in the parent DataAccess Class, so it knows that its the Users subclass that's called it?

Anton · Accepted Answer · 2020-02-18 16:29:28Z

1

You can use generic class


export class DataAccess<T> {
  columns: { [P in keyof T]: string };
  tableName: string;
  constructor({ tableName }: { tableName: string }) {
    this.tableName = tableName;
  }
  async getWhere(where: Partial<T>, { single = false } = {}) {
    const options: { limit?: number } = {};
    if (single) {
      options.limit = 1;
    }
    const results = await someDBobj[this.tableName].find(where, options);
    if (!single) {
      return results;
    }
    return results.length ? results[0] : null;
  }
}

import { DataAccess } from "./data-access";

type UsersTable = {
  id: string;
  email: string;
  password: string;
  createdAt: Date;
};

export default class Users extends DataAccess<UsersTable > {
  constructor() {
    super({
      tableName: "users"
    });
    this.columns = {
      id: "id",
      email: "email",
      password: "password",
      createdAt: "createdAt"
    };
  }
}

edited Feb 18, 2020 at 16:29

answered Feb 18, 2020 at 16:16

Anton

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1 Comment

Josh Wulf Over a year ago

async getWhere(where: Partial<T>, { single = false } = {}) {, I think...

Josh Wulf · Accepted Answer · 2020-02-18 16:21:43Z

It looks like you are only extending the class to get specialisation of the data shape. In that case, you could do something like this:

class DataAccess<T> {
  columns: { [P in keyof T]: string };
  tableName: string;
  constructor({ tableName, columns }: { tableName: string, columns: { [P in keyof T]: string } }) {
    this.tableName = tableName;
    this.columns = columns;
  }
  async getWhere(where: Partial<T>, { single = false } = {}) {
    const options: { limit?: number } = {};
    if (single) {
      options.limit = 1;
    }
    const results = await someDBobj[this.tableName].find(where, options);
    if (!single) {
      return results;
    }
    return results.length ? results[0] : null;
  }
}

type UsersTable = {
  id: string;
  email: string;
  password: string;
  createdAt: Date;
};

const user = new DataAccess<UsersTable>({tableName: ""users", columns: {
  id: "id",
  email: "email",
  password: "password",
  createdAt: "createdAt"
};})

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