Array reduce transformation

85.5% acceptance

Mar 2, 2026

765

Given an integer array nums, a reducer function fn, and an initial value init, return the final result obtained by executing the fn function on each element of the array, sequentially, passing in the return value from the calculation on the preceding element. This result is achieved through the following operations: val = fn(init, nums[0]), val = fn(val, nums[1]), val = fn(val, nums[2]), ... until every element in the array has been processed. The ultimate value of val is then returned. If the length of the array is 0, the function should return init. Please solve it without using the built-in Array.reduce method.

Solution

TypeScript

Time O(n)

Space O(1)

LeetCode

solution.ts

type Fn = (accum: number, curr: number) => number;

function reduce(nums: number[], fn: Fn, init: number): number {
  let val = init;
  for (let i = 0; i < nums.length; i++) {
  val = fn(val, nums[i]);
  }
  return val;
}