为什么这个合并排序代码可以在python中工作，而不能在javascript中工作？

def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    
    mid = len(arr) // 2
    left = merge_sort(arr[:mid])
    right = merge_sort(arr[mid:])
    
    return merge(left, right)

def merge(left, right):
    result = []
    i = j = 0
    
    while i < len(left) and j < len(right):
        if left[i] < right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1
    
    result.extend(left[i:])
    result.extend(right[j:])
    
    return result

function mergeSort(arr) {
    if (arr.length <= 1) {
        return arr;
    }
    
    const mid = Math.floor(arr.length / 2);
    const left = arr.slice(0, mid);
    const right = arr.slice(mid);
    
    return merge(mergeSort(left), mergeSort(right));
}

function merge(left, right) {
    const result = [];
    let i = 0;
    let j = 0;
    
    while (i < left.length && j < right.length) {
        if (left[i] < right[j]) {
            result.push(left[i]);
            i++;
        } else {
            result.push(right[j]);
            j++;
        }
    }
    
    return result.concat(left.slice(i)).concat(right.slice(j));
}