promise-reduce-errors.js

// Output down

const arr = [
  { value: 1 },
  { value: 2 },
  { value: 3 },
  undefined,
  { value: 5 },
  { value: 6 }
];

const doubledValue = x => x.value * 2;

const summed = arr.reduce(
  (acc, entry) =>
    acc
      .then(sum => {
        const newSum = sum + entry.value;
        console.log(`current sum: ${newSum}, current value: ${entry.value}`);

        return newSum;
      })
      .catch(console.log), // this is problematic. since the error is handled here, it does not propagate further
  Promise.resolve(0)       // either do not catch error here, or if you need to, explicitly return Promise.reject()
);

summed
  .then(v => console.log(`Success! Final sum: ${v}`))
  .catch(err => console.log(`Summing failed due to an error: ${err}`));

/* OUTPUT

"current sum: 1, current value: 1"

"current sum: 3, current value: 2"

"current sum: 6, current value: 3"

[object Error] {}

"current sum: NaN, current value: 5"

"current sum: NaN, current value: 6"

"Success! Final sum: NaN"

*/