app-promises.js

const users = [{
  id: 1,
  name: 'Andrew',
  schoolId: 101
},{
  id: 2,
  name: 'Jessica',
  schoolId: 999
}];

const grades = [{
  id: 1,
  schoolId: 101,
  grade: 86
}, {
  id: 2,
  schoolId: 999,
  grade: 100
}, {
  id: 3,
  schoolId: 101,
  grade: 80
}];

const getUser = (id) => {
  return new Promise((resolve, reject) => {
    const user = users.find((user) => user.id === id);

    if (user) {
      resolve(user)
    } else {
      reject(`Unable to find user with id of ${id}.`);
    }
  });
};

const getStatus = (userId) => {
  var user;
  return getUser(userId).then((tempUser) => {
    user = tempUser;
    return getGrades(user.schoolId);
  }).then((grades) => {
    var avg = 0;
    if (grades.length > 0) {
      avg = grades.map((grade) => grade.grade).reduce((a, b) => a+b)/grades.length;
    }
    return `${user.name} has a ${avg}% in the class.`;
  })
};

const getStatusAlt = async (userId) => {
  const user = await getUser(userId);
  const grades = await getGrades(user.schoolId);
  var avg = 0;
  if (grades.length > 0) {
    avg = grades.map((grade) => grade.grade).reduce((a, b) => a+b)/grades.length;
  }
  return `${user.name} has a ${avg}% in the class.`;
};

getStatusAlt(1).then((status) => {
  console.log(status);
}).catch((e) => {
  console.log(e);
});
// getUser(3).then((user) => {
//   console.log(user);
// }).catch((e) => {
//   console.log(e);
// });

const getGrades = (schoolId) => {
  return new Promise((resolve, reject) => {
    resolve(grades.filter((grade) => grade.schoolId === schoolId));
  });
};

// getGrades(101).then((grade) => {
//   console.log(grade);
// }).catch((e) => {
//   console.log(e);
// });

// getStatus(1).then((status) => {
//   console.log(status);
// }).catch((e) => {
//   console.log(e);
// });