Description

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Intermediate · 12 min

Reduce and group data

Use an explicit initial accumulator to calculate totals and counts.

Choose an initial value

reduce() combines items into one result. Providing an initial value also defines the result for an empty array.

const prices = [4, 6, 3];
const total = prices.reduce((sum, price) => sum + price, 0);
console.log(total);
console.log([].reduce((sum, value) => sum + value, 0));

Output

13
0

Count repeated values

An accumulator can be an object or Map. A Map avoids treating arbitrary input strings as object property names.

const counts = ["tea", "coffee", "tea"].reduce((map, name) => {
  map.set(name, (map.get(name) ?? 0) + 1);
  return map;
}, new Map());
console.log(counts.get("tea"));

Output

2

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Total quantities

Use reduce() to add the quantity of every item. Set total and display it.

Practice