Promises and Fetch

Promises

pg-promise: using promises with DB queries

The Peppr app uses promises when doing DB queries, such as:






const getUser = email => {
  return db.query('SELECT * FROM users WHERE email = $1', [email])
  .then(user => user[0])
  .catch(console.log)
}

This is possible without constructing a new Promise because the module pg-promise has been required in their db_connection.js file. It allows us to handle the result from a DB query once it has finished, without having to use callbacks.

The Peppr app has a good example of connection promises together in their auth.js file:



























//this requires the DB queries like getUser above
const queries = require('./queries'); 

queries
  .getUser(email)
  .then(user => {
    //a new promise is created
    return new Promise((resolve, reject) => {
      if(user){
        res.status(422).send({ error: 'Email is in use. Please login'});
          //the user already exists, so an error is thrown using reject
        reject('Email is in use. Please sign in');
      } 
        //the user doesn't exist yet, so their password is hashed
      else resolve(hashPassword(password));
    })
  })
  .then(hash => {
    return queries.addUser(name, email, hash)
  })
  .then (user => {
    //as this is the last then, it doesn't need to be asynchronous
    res.json({ token: userToken(user)});
  })
  .catch(console.log)
}

request-promise module

To turn get requests (e.g. an external API call) into a promise easily, there is the request-promise module.










const rp = require('request-promise');

rp('https://pokeapi.co/api/v2/pokemon/pikachu/')
    .then(function (htmlString) {
        // Process html...
    })
    .catch(function (err) {
        // Crawling failed...
    });

This also means you won't need to chunk-ify/parse the body. It comes back fully-formed and ready to be handled.

Use promise in frontend

Since it is not supportes by all browser, use promise-polyfill

Example of using promises in testing

db_build.js:






const runDbBuild = () => {
  return db.any(build)
    .then(res => console.log('res', res))
    .catch(e => console.error('error', e));
};

test:












test('check parent exists', (t) => {
  runDbBuild().then(() => {
    const email = 'k@a.com';
    return checkParent(email)
  }).then((queryRes) => {
    t.equal(queryRes[0].case, true, 'If parent exists then check_parent should return true- promise');
    t.end();
  }).catch((err) => {
    throw err;
  })
})
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