factorial promises

fact.promises.js

class ZeroValue extends Error {}
class OtherValue extends Error {}

function resolveValue(val) {
  return Promise.resolve().then(() => {
    if (!val) throw new ZeroValue();
    throw new OtherValue(val);
  });
}

function factorial(n) {
  return resolveValue(n)
    .catch(ZeroValue, () => 1)
    .catch(OtherValue, val => factorial(val.message — 1).then(prev => val.message * prev))
}

factorial(4).then(console.log);