Currently, I have setup my code as follows:

import pgPromise from "pg-promise";

const pgp = pgPromise();

export const createDBConnection = async () => {
    const db = pgp({
        host: "",
        ssl: false,
        user: "",
        password: "",
        port: 3306,
        max: 25
    });

    try {
        const connection = await db.connect();
        connection.done();
    } catch (err) {
        console.error(err)
    }

    return db;
}

and my test as follows:

import { createDBConnection } from ".";

jest.mock('pg-promise');

describe('db', () => {
    it('initialises with correct parameters', async () => {
        await createDBConnection();
        ...
    });
});

Unfortunately, the test fails with:

TypeError: pgp is not a function

The following works:

import { createDBConnection } from ".";

jest.mock('pg-promise', () => () => jest.fn());

describe('db', () => {
    it('initialises with correct parameters', async () => {
        await createDBConnection();
    });
});

and

const pgpMock = jest.fn().mockImplementation(() => ({
    connection: jest.fn().mockReturnValue({ end: jest.fn() }),
    connect: jest.fn().mockReturnValue({ done: jest.fn() })
}));

jest.mock('pg-promise', () => () => pgpMock);

import { createDBConnection } from '.';

describe('db', () => {
    it('initialises with correct parameters v1', async() => {
       await createDBConnection();
       expect(pgpMock).toHaveBeenCalled();
    });
});

But I am looking to use jest.mocked so I can check that pgp function is called with the correct parameters since these might come from an external configuration.

import pgPromise from 'pg-promise';
import { createDBConnection } from '.';

jest.mock('pg-promise');

describe('db', () => {
    it('initialises with correct parameters v1', async() => {
        const pgpMock = jest.fn().mockImplementation(() => ({
            connection: jest.fn().mockReturnValue({ end: jest.fn() }),
            connect: jest.fn().mockReturnValue({ done: jest.fn() })
        }));

        
       jest.mocked(pgPromise).mockReturnValue(() => pgpMock);
       await createDBConnection();
       expect(pgpMock).toHaveBeenCalled();
    });
});

Which makes pgp to be undefined when called.

I am using the following libraries:

"pg-promise": "^11.9.0"
"jest": "^29.7.0",
"ts-jest": "^29.2.0",
"typescript": "^5.5.3"

I expected to be able to mock pgPromise implementation to return a mock function so when calling pgp({}) I can assert its parameters.