// my-api.ts
import axios from 'axios';

export class MyApi extends axios.Axios {
  constructor(baseURL: string) {
    super({
      baseURL,
    });

    this.interceptors.response.use(
      (data)=> data,
      (error)=> {
         // Never enters here
          return Promise.reject({
          ...error,
          customError: error.message,
        });
      },
    );
  }
}

// my-api.spec.ts
import type { AxiosInstance } from 'axios';
import MockAdapter from 'axios-mock-adapter';

import { MyApi } from './my-api';

describe('my-api', () => {
  // NOTE: simulate an instance of the api
  const api = new MyApi('http://localhost');
  const apiMock = new MockAdapter(api as AxiosInstance);

  it('should handler request error interceptor', async () => {
    try {
      apiMock.onGet('/fake_data').reply(404);
      await api.request({ method: 'get', url: '/fake_data' });
      
      // NOTE: this should never happen since endpoint fails and enters directly on catch
      expect(1).toBe(2);
    } catch (error) {
      const { customError } = error as { customError: string };
      expect(customError).toBe('example error');
    }
  });
});