HardArray Methods💻 Output Question

reduce to group and aggregate

💡

Hint

reduce is ideal for grouping and aggregating — a single pass over an array to build any shape of output.

What does this output?

const orders = [
  { cat: 'A', val: 10 },
  { cat: 'B', val: 20 },
  { cat: 'A', val: 30 },
];
const totals = orders.reduce((acc, o) => {
  acc[o.cat] = (acc[o.cat] || 0) + o.val;
  return acc;
}, {});
console.log(totals.A);
console.log(totals.B);

Correct Output

40
20

Why this output?

Explanation: reduce builds a totals map. A: 10+30=40. B: 20.

Key Insight: reduce is ideal for grouping and aggregating — a single pass over an array to build any shape of output.

What does this output?

Correct Output

Why this output?

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