JavaScript Fold with reduce: Accumulate a Checked Summary

The task

Summarize parcel count and total weight from fictional records. Each callback returns a new summary instead of relying on an accidental callback side effect.

Runnable Node 22 example

const assert = require('node:assert/strict');

const parcels = [{kg: 2}, {kg: 5}, {kg: 1}];
const summarize = items => items.reduce((summary, item) => ({
  count: summary.count + 1,
  kg: summary.kg + item.kg
}), {count: 0, kg: 0});
assert.deepEqual(summarize(parcels), {count: 3, kg: 8});
assert.deepEqual(summarize([]), {count: 0, kg: 0});
assert.throws(() => [].reduce((a, b) => a + b), TypeError);
console.log(JSON.stringify(summarize(parcels)));

Expected output and explanation

The accumulator starts at zero parcels and zero kilograms. Each record increments the count and adds its weight. Returning the updated summary carries that value into the next callback, producing three parcels totaling eight kilograms.

The empty list returns the provided initial summary without a callback. The separate assertion demonstrates the failure of reducing an empty array without an initial value. This is one reason the initial accumulator belongs in the contract rather than being an afterthought.

Pitfall to check

An omitted callback return changes the next accumulator to undefined and usually breaks the calculation. Validate units and input types before using a summary in a real workflow; this fixture contains known numeric kilogram values.