Get errors and results from a parallel asynchronous call

promise_all.js

// Wrapper to get results & errors in parallel: 
// Here we could override the original Promise.all method but due to this is just merely a test, we'll just leave it aside.
function promiseAll(array, combine) {
  return Promise.all(array.map(p => p.catch(e => e)))
    .then(values => {
      if (combine) {
        return values;
      }
    
      let errors = values.reduce((target, value) => {
      
        if (value instanceof Error) {
          target.push(value);
        };
      
        return target;
      }, []);

      if (errors.length) {
        throw errors;
      } else {
        return values;
      };
    });
}

// Helper for the proof of concept:
function create({ message, time, error }) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (error) {
        reject(new Error(message));
      } else {
        resolve(message);
      }
    }, time || 1000);
  });
}

// Invoke (not combined):
promiseAll([
  create({ message: 'OK 1!' }),
  create({ message: 'OK 2!' }),
  create({ message: 'NO 3!', error: true, time: 3000 }),
  create({ message: 'OK 4!' }),
  create({ message: 'NO 5!', error: true, time: 2000 })
])
.then(result => {
  console.log('PASS:', result);
})
.catch(error => {
  console.log('FAIL:', error);
})

/* output: (2 errors)
FAIL: (2) [Error: NO 3!
    at setTimeout (<anonymous>:25:16), Error: NO 5!
    at setTimeout (<anonymous>:25:16)]
*/

// Invoke (combined):
// Using true as second parameter we will get all the errors and results and the promise will be resolved.
// So, here we don't need a catch block.
promiseAll([
  create({ message: 'OK 1!' }),
  create({ message: 'OK 2!' }),
  create({ message: 'NO 3!', error: true, time: 3000 }),
  create({ message: 'OK 4!' }),
  create({ message: 'NO 5!', error: true, time: 2000 })
], true)
.then(result => {
  console.log(result);
});

/* output: (2 errors & 3 results)
["OK 1!", "OK 2!", Error: NO 3!
    at setTimeout (<anonymous>:29:16), "OK 4!", Error: NO 5!
    at setTimeout (<anonymous>:29:16)]
*/

// Method #3 (Tradition way) it just throw the last error found:
Promise.all([
  create({ message: 'OK 1!' }),
  create({ message: 'OK 2!' }),
  create({ message: 'NO 3!', error: true, time: 3000 }),
  create({ message: 'OK 4!' }),
  create({ message: 'NO 5!', error: true, time: 2000 })
])
.then(result => {
  console.log('PASS:', result);
})
.catch(error => {
  console.log('FAIL:', error);
})

/* output: (fail with the last error found)
FAIL: Error: NO 5!
    at setTimeout (<anonymous>:5:16)
*/