testing private methods in typescript with jest

It looks like you are wanting to verify that handleError gets called when build runs.

Private methods are compiled to normal JavaScript prototype methods, so you can use the any type to let the spy creation pass through the TypeScript type checking.

Here is a highly simplified example:

class OrderBuilder {
  public build() {
    this.handleError()
  }
  private handleError() {
    throw new Error('missing ... field in order')
  }
}

describe('Order Builder', () => {
  it('should test the handleError', () => {
    const handleErrorSpy = jest.spyOn(OrderBuilder.prototype as any, 'handleError');
    const orderBuilder = new OrderBuilder()
    expect(() => orderBuilder.build()).toThrow('missing ... field in order');  // Success!
    expect(handleErrorSpy).toHaveBeenCalled();  // Success!
  });
});