reduce.js

Array.prototype.reduceCustom = function(callback, initialValue) {
    const array = this

    if (!array.length) {
        if (initialValue) {
            return initialValue
        }

        throw new Error('TypeError: Reduce of empty array with no initial value')
    }

    if (!initialValue && array.length === 1) {
        return array[0]
    }

    let accumulator = initialValue ?? array[0]
    const startIndex = initialValue ? 0 : 1

    for (let i = startIndex, length = array.length; i < length; i++) {
        accumulator = callback(accumulator, array[i], i, array)
    }

    return accumulator
}

Array.prototype.flatCustom = function(depth = 1) {
    const array = this

    const makeFlat = (a, c) => a.concat(
        Array.isArray(c) && depth > 1 
            ? c.flatCustom(depth - 1) 
            : c
    )

    return array.reduceCustom(makeFlat, [])
}

// Reducer tests

const reduceTest = [1, 2, 3, 4, 5]

const sumCallback = (a, c) => (a + c)

console.log('Reducer - Sum', reduceTest.reduce(sumCallback))
console.log('CustomReducer - Sum', reduceTest.reduceCustom(sumCallback))

console.log('Reducer - Empty with Initial Value', [].reduce(sumCallback, 1))
console.log('CustomReducer - Empty with Initial Value', [].reduceCustom(sumCallback, 1))

console.log('Reducer - One Item with Initial Value', [2].reduce(sumCallback, 1))
console.log('CustomReducer - One Item with Initial Value', [2].reduceCustom(sumCallback, 1))

const commonResult = (a, c, i) => {
    a.push({c, i})
    return a
}

console.log('Reducer - Result', [2, 3].reduce(commonResult, []))
console.log('CustomReducer - Result', [2, 3].reduceCustom(commonResult, []))

// Flat Test

console.log('Flat', [2, 3, [4, 5], 1].flat())
console.log('FlatCustom', [2, 3, [4, 5], 1].flatCustom())

console.log('Flat - With depth 2', [2, 3, [4, 5, [6, 7, [8, 9]]], 1].flat(2))
console.log('flatCustom - With depth 2', [2, 3, [4, 5, [6, 7, [8, 9]]], 1].flatCustom(2))

console.log('Flat - With depth 3', [2, 3, [4, 5, [6, 7, [8, 9]]], 1].flat(3))
console.log('flatCustom - With depth 3', [2, 3, [4, 5, [6, 7, [8, 9]]], 1].flatCustom(3))