Array.reduce()

The reduce() method sequentially applies a callback to each element of the array, storing the intermediate result in an accumulator and returning a single final value.

Signature

arr.reduce(callback(accumulator, currentValue, index, array), initialValue);

How it works

1. Accumulator — stores the result of the previous function call.
2. Current Value — the element currently being processed.
3. If initialValue is specified, it is used as the initial value of the accumulator. If not specified — reduce() will take the first element of the array as the start, and iteration will begin from the second element.
4. In each iteration, the result of the callback is assigned to the accumulator.
5. After completing all iterations, the final value of the accumulator is returned.

Example

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((acc, val) => acc + val, 0);
// acc: 0 → 1 → 3 → 6 → 10
// sum = 10

Key Features

• Does not mutate the original array.
• Works with any data type (numbers, strings, objects).
• Suitable for:
  • summing and aggregating data,
  • counting element frequency,
  • grouping by properties,
  • transforming arrays into objects.