Description

JavaScript provides the built-in Array.prototype.reduce() method that executes a reducer function on each element of the array, resulting in a single output value.

reduce is one of the most powerful array methods because it handles accumulator logic, optional initial values, sparse arrays, and edge-case errors.

In this challenge, you’ll implement your own version of reduce that behaves exactly like the native one.

Your Task

Implement a method Array.prototype.myReduce that:

  • Accepts a reducer callback function
  • Optionally accepts an initialValue
  • Iterates over the array from left to right
  • Accumulates a single return value
  • Behaves exactly like Array.prototype.reduce

Example

Input

const nums = [1, 2, 3];
const sum = nums.myReduce((acc, cur) => acc + cur, 0);
console.log(sum);

Output

6

More Examples

[1, 2, 3].myReduce((a, b) => a + b);
[1, 2, 3].myReduce((a, b) => a * b, 1);
[, 2, 3].myReduce((a, b) => a + b);
["a", "b", "c"].myReduce((acc, cur) => acc + cur);

Expected Behavior

  • Uses initialValue if provided
  • If no initialValue, uses the first defined element
  • Skips sparse array holes
  • Passes (accumulator, currentValue, index, array) correctly
  • Throws TypeError for empty array without initial value
  • Does not mutate the original array

Constraints

  • Do not use Array.prototype.reduce
  • Time complexity should be O(n)
  • Space complexity should be O(1), excluding call stack