44Implement asyncReduce() with sequential execution

Implement asyncReduce(items, reducer, initialValue). It works like Array.prototype.reduce, but the reducer(accumulator, item, index) may be async, and each call starts only after the previous one has finished, because each step needs the previous result.

// Apply database migrations one after another.
const version = await asyncReduce(migrations, async (current, migration) => {
  await migration.run();
  return migration.version;
}, 0);

Requirements

• Run the reducer calls strictly one at a time, in order.
• Items may themselves be promises; await each item before passing it on.
• Without an initial value, the first item is the starting accumulator. With an empty input and no initial value, reject with a TypeError.
• An explicit initialValue of undefined still counts as given.
• If a call rejects, reject and do not call the reducer again.