02-promise-allsettled

Exercise 02 - Batch sensor read with allSettled

The scenario

The park has dozens of zone sensors — temperature, humidity, motion. On any given read cycle, half of them might be offline. Using Promise.all would reject the entire batch if a single sensor fails. The dashboard needs every result — successes and failures — so it can show a status for each sensor.

You'll use Promise.allSettled to call every sensor, then normalise the results into a uniform shape the dashboard can render.

What you will build

summarizeSensorBatch(sensorFns) — in starter/summarize-batch.js

sensorFns is an array of functions that each return a promise (simulating a sensor read). Call every one, wait for all to settle, and return an array of summary objects:

• For successes: { status: 'fulfilled', value }
• For failures: { status: 'rejected', reason } where reason is a string (use String(err?.message ?? err))

const results = await summarizeSensorBatch([
  () => Promise.resolve('temperature: 22C'),
  () => Promise.reject(new Error('sensor offline')),
  () => Promise.resolve('humidity: 45%'),
]);
// [
//   { status: 'fulfilled', value: 'temperature: 22C' },
//   { status: 'rejected', reason: 'sensor offline' },
//   { status: 'fulfilled', value: 'humidity: 45%' },
// ]

Non-Error rejections (e.g. a plain string) should also be converted to a string reason.

Getting started

Open starter/summarize-batch.js. Implement the function. Then run:

node starter/index.js

Verify

cd starter && pnpm install && pnpm test

The tests check mixed fulfilled/rejected results and non-Error rejection values.

Hints

• Call each function first to get promises: sensorFns.map(fn => fn()).
• Promise.allSettled(promises) returns an array of { status, value } or { status, reason } objects.
• Map over the settled results to normalise the reason to a string.
• String(reason?.message ?? reason) handles both Error objects and plain string rejections.

Reference solution

solution/summarize-batch.js