try mock sendgrid

sendgrid.js

'use strict';

const sendgrid = require('sendgrid')(process.env.MAIL_KEY || 'may secrete key');

function Mailer () {
  this.sendMail =  body => {
    if(!this.verifyMailbody(body)) {
      throw new TypeError('invalid body');
    }

    return new Promise((resolve, reject) => {
      return sendgrid.send({
        body
      }, (err, json) => {
        err ? reject(err) : resolve(json);
      });
    });
  };

  this.verifyMailbody = body => {
    return Object
      .keys(body)
      .map(verifyProperts)
      .reduce(b => b);
  };

  const propertys = ['to', 'from', 'subject', 'text'];

  function verifyProperts(body, index) {
    return propertys[index] === body;
  }
}

module.exports = Mailer;
///testes

'use strict';

const chai = require('chai');
const expect = chai.expect;
const chaiAsPromise = require('chai-as-promised');
chai.use(chaiAsPromise);
const Mailer = require('../mailer/mailer');
const sinon = require('sinon');
require('sinon-as-promised');

describe('mailer', () => {
  let mail = null;
  let mailer = null;
  before(() => {
    mailer = new  Mailer();
    mail = {
      to      : 'example@example.com',
      from    : 'other@example.com',
      subject : 'Hello World',
      text    : 'My first email through SendGrid.'
    };
  });
  describe('mailer', () => {
    it('should be mailer has method send mail', () => {
      expect(mailer).to.have.property('sendMail');
    });

    it('should be mailer mail has method then', () => {
      expect(mailer.sendMail(mail)).to.be.instanceof(Promise);
    });
    
    it('should be throw error if body missing properts', () => {
      expect(() => {
        mailer.sendMail();
      }).to.throw(TypeError, /body no has to,from,subject,text/);
    });

    it('should be method sendiMail recive a body', () => {
      const spy  = sinon.spy(mailer, 'sendMail');
      spy.withArgs(mail);
      mailer.sendMail(mail);
      expect(spy.withArgs(mail).calledOnce).to.be.true;
    });

    it('should be method sendiMail throw if invalid body ', () => {
      const mailFail = {
        a:       'example@example.com',
        from:     'other@example.com',
        subject:  'Hello World',
        text:     'My first email through SendGrid.'
      };
      expect(function () {
        mailer.sendMail(mailFail);
      }).to.throw(TypeError, /^invalid body$/);
    });

    it('should be method sendMail send a mail with success', done => {
      const mailer = new  Mailer();

      const mock = sinon.mock(mailer);
      const success = {success : 'mail success'};

      mock.expects('sendMail')
        .once()
        .withArgs(mail)
        .resolves(success);

      expect(mailer.sendMail(mail))
        .to
        .eventually
        .equal(success)
        .notify(done);
    });
  });
});