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

function identifyTests() {
  //* constructor
  (new Identify(): Identify);

  const i: Identify = new Identify();

  //* add
  i.add('foo', 1);
  i.add('bar', -10);

  // $ExpectError: add requies a name as first parameter
  i.add(15, 21);

  // $ExpectError: add requies a number as second parameter
  i.add('foo', 'bar');

  //* append
  i.append('foo', 1);
  i.append('foo', [1]);

// @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 identifyTests() {
  //* constructor
  (new Identify(): Identify);

  const i: Identify = new Identify();

  //* add
  i.add('foo', 1);
  i.add('bar', -10);

  // $ExpectError: add requies a name as first parameter
  i.add(15, 21);

  // $ExpectError: add requies a number as second parameter
  i.add('foo', 'bar');

  //* append
  i.append('foo', 1);
  i.append('foo', [1]);
  i.append('foo', 'bar');
  i.append('foo', ['bar']);

export const setupUser = userInfo => {
  const { username: userId } = userInfo;
  amplitude.getInstance().setUserId(userId);
  let userIdentity = new amplitude.Identify();
  Object.keys(userInfo).forEach(function(key) {
    userIdentity.set(key, userInfo[key]);
  });
  amplitude.getInstance().identify(userIdentity);
};