How would you cover an online purchase system via YuKassa (webhook from a third-party service) with automated tests?

For automated testing of the purchase system via Yookassa using webhooks, I would distinguish several levels:

1. Unit tests — checking the logic of processing incoming webhook events, such as data validation and payment status handling.

2. Integration tests — testing interaction with the external Yookassa service through a mock server that simulates sending webhook requests with different scenarios (successful payment, cancellation, error).

3. E2E tests — testing the complete purchase scenario in the system, including order creation, payment, and receiving webhook for payment confirmation.

Example of an integration test in Node.js using a mocking library:

const request = require('supertest');
const app = require('../app'); // express application

describe('Yookassa Webhook', () => {
  it('Handles successful payment', async () => {
    const webhookPayload = {
      // example webhook data
      event: 'payment.succeeded',
      object: {
        id: 'payment_id',
        status: 'succeeded',
        amount: 1000
      }
    };

    const response = await request(app)
      .post('/webhook/yookassa')
      .send(webhookPayload)
      .set('Content-Type', 'application/json');

    expect(response.statusCode).toBe(200);
    // Additional checks of order status in the database
  });
});

Thus, test coverage will ensure the reliability of payment processing and the correct operation of the system under various scenarios.