Array.prototype.reduce

easycodingJavaScriptArraysPrototypes~10 min

Asked at Google, Meta, Amazon

Overview

Implement reduce(array, callback, initialValue) that reduces an array to a single value.

Constraints

callback(accumulator, currentValue, index, array)
If no initialValue, use first element as accumulator.

Examples

reduce([1,2,3,4], (acc, v) => acc + v, 0); // 10

Solution

Reveal solution

function reduce(array, callback, initialValue) {
  let acc = initialValue !== undefined ? initialValue : array[0];
  const start = initialValue !== undefined ? 0 : 1;
  for (let i = start; i < array.length; i++) acc = callback(acc, array[i], i, array);
  return acc;
}

array-prototype-reduce.js

Array.prototype.reduce

easycodingJavaScriptArrays

Overview

Implement reduce(array, callback, initialValue) that reduces an array to a single value.

Constraints

callback(accumulator, currentValue, index, array)
If no initialValue, use first element as accumulator.

Examples

reduce([1,2,3,4], (acc, v) => acc + v, 0); // 10

Solution

Reveal solution

function reduce(array, callback, initialValue) {
  let acc = initialValue !== undefined ? initialValue : array[0];
  const start = initialValue !== undefined ? 0 : 1;
  for (let i = start; i < array.length; i++) acc = callback(acc, array[i], i, array);
  return acc;
}

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