How to use the amplitude-js.Revenue function in amplitude-js

function revenueTests() {
  //* constructor
  (new Revenue(): Revenue);

  const r: Revenue = new Revenue();

  //* setProductId
  r.setProductId('pid');

  // $ExpectError: setProductId only accepts strings
  r.setProductId(5);

  // $ExpectError: setProductId only accepts strings
  r.setProductId();

  //* setQuantity
  r.setQuantity(5);
  r.setQuantity();

// @flow

/* eslint-disable no-unused-vars, no-undef, no-console */

import amplitude, { Identify, Revenue } from 'amplitude-js';
import type { AmplitudeClient } from 'amplitude-js';

/**
 * Common instances
 */

const client = amplitude.getInstance();
const identify = new Identify();
const revenue = new Revenue();
const callback = (responseCode: number, responseBody: string) => void 0;

/**
 * ALL THE TESTS START FROM HERE
 */

function getInstanceTest() {
  (amplitude.getInstance(): AmplitudeClient);
  (amplitude.getInstance('name'): AmplitudeClient);

  // $ExpectError: getInstance only accepts strings
  amplitude.getInstance(5);

  // $ExpectError: getInstance only accepts strings
  amplitude.getInstance(new Date());
}

function revenueTests() {
  //* constructor
  (new Revenue(): Revenue);

  const r: Revenue = new Revenue();

  //* setProductId
  r.setProductId('pid');

  // $ExpectError: setProductId only accepts strings
  r.setProductId(5);

  // $ExpectError: setProductId only accepts strings
  r.setProductId();

  //* setQuantity
  r.setQuantity(5);
  r.setQuantity();

  // $ExpectError: setProductId only accepts numbers
  r.setQuantity('foo');