APICotacao.js

const axios = require('axios');
const getURL = data => `https://olinda.bcb.gov.br/olinda/servico/PTAX/versao/v1/odata/CotacaoDolarDia(dataCotacao=@dataCotacao)?@dataCotacao='${data}'&$top=100&$format=json&$select=cotacaoCompra,cotacaoVenda,dataHoraCotacao`;

const getCotacaoAPI = url => axios.get(url);
const extractCotacao = res => res.data.value[0].cotacaoVenda;
const getToday = () => {
  const today = new Date();
  return (today.getMonth()+1)+'-'+today.getDate()+ '-'+today.getFullYear();
}
const getCotacao =  async () => {
  try {
      const today = getToday();
      const url = getURL(today);
      const res = await getCotacaoAPI(url);
      // const res = await getCotacaoAPI('4-12-2019');
      const cotacao = extractCotacao(res);
      return cotacao;
  } catch(err) {
    console.log(err);
    return '';
  } 
} 

module.exports = {
  getCotacaoAPI,
  getCotacao,
  extractCotacao
}




Arquivo de Teste

const api = require('../OlindaAPI');
const axios = require('axios');
jest.mock('axios');


describe('OlindaAPI', () => {
  it('should get cotacao API', () => {
    const response = {
      data: {
        value: [
          { cotacaoVenda: 3.90 }
        ],
      }
    };
    axios.get.mockResolvedValue(response);
    api.getCotacaoAPI('url').then( resp => {
      expect(resp).toEqual(response);
      console.log(axios.get.mock);
      expect(axios.get.mock.calls[0][0]).toBe('url');
    });
  }); 
});