Javascript merge sort implementation

merge-sort.js

/* usage example */
console.log(
    mergeSort(
        [1, 8, 6, 8, 6, 4, 2, 10, 89, 5456, 55],
        (a, b) => {
            if (a > b) {
                return -1
            } else if (a < b) {
                return 1
            }

            return 0
        }
    )
)

/* merge sort implementation */
function mergeSort(list, compareFn) {
    if (list.length <= 1) {
        return list
    }

    const length = list.length
    const left = []
    const right = []

    for (let index = 0; index < length; index++) {
        if (index < length / 2) {
            left.push(list[index])
        } else {
            right.push(list[index])
        }
    }

    return mergeList(mergeSort(left, compareFn), mergeSort(right, compareFn), compareFn)
}

function mergeList(left, right, compareFn) {
    const result = []
    let a, b

    while (left.length > 0 && right.length > 0) {
        a = left[0]
        b = right[0]

        if (compareFn(a, b) <= 0) {
            result.push(a)
            left.shift()
        } else {
            result.push(b)
            right.shift()
        }
    }

    while (left.length > 0) {
        result.push(left.shift())
    }

    while (right.length > 0) {
        result.push(right.shift())
    }

    return result
}