async/await example

example.js

function isValidEmail(email) {
  return email.match(/^[^\s@]+@[^\s@]+\.[^\s@]+$/);
}

function getUserByEmail(email) {
  return new Promise((resolve) => {
    setTimeout(() => {
      // keep rejections for exceptions only
      if (email !== 'roy@someemail.com') {
        return resolve(null);
      }
      resolve({
        name: 'roy',
        email: 'roy@someemail.com',
        password: 'abc123$'
      });
    }, 1000);
  });
}

function compare(p1, p2) {
  return new Promise((resolve) => {
    setTimeout(() => {
      // keep reject for failures
      resolve(p1 === p2);
    }, 1000);
  });
}

async function getByEmailAndPassword({ email, password }) {
  const isValid = await isValidEmail(email);
  if (!isValid) {
    return {
      token: 'INVALID_EMAIL'
    };
  }

  const user = await getUserByEmail(email);
  if (!user) {
    return {
      token: 'EMAIL_AND_PASSWORD_MISMATCH'
    };
  }

  const isCorrect = await compare(password, user.password);
  if (!isCorrect) {
    return { token: 'EMAIL_AND_PASSWORD_MISMATCH' };
  }
  return user;
}

getByEmailAndPassword({ email: 'roy@someemail.com', password: 'abc123$' }).then((result) => {
  console.log('result', result);
}, (err) => console.error(err));