countTotal調用示例：

• 模板數組

let arr = [
    {id: 0, price: 199.88},
    {id: 1, price: 299.88},
    {id: 2, price: 399.88}
];
console.log(countTotal(arr, 'price')); //899.64

• 計算函數

//計算對象數組中某個屬性合計
function countTotal(arr, keyName) {
    let $total = 0;
    $total = arr.reduce(function (total, currentValue, currentIndex, arr){
        return currentValue[keyName] ? (total + currentValue[keyName]) : total;
    }, 0);
    return $total;
}