knex/test-tsd/querybuilder.test-d.ts

import { knex } from '../types';
import { expectType } from 'tsd';
import { clientConfig } from './common';

const knexInstance = knex(clientConfig);

// Use:
// import { Knex } from 'knex'

// This would be `declare module 'knex'` in runtime code
declare module '../types' {
  namespace Knex {
    interface QueryBuilder {
      customSelect<TRecord, TResult>(
        value: number
      ): Knex.QueryBuilder<TRecord, TResult>;
    }
  }
}

knex.QueryBuilder.extend('customSelect', function (value: number) {
  return this.select(this.client.raw(`${value} as value`));
});

const main = async () => {
  expectType<number[]>(await knexInstance('users').customSelect<any, number[]>(42));
};