es6求平均值 -- reduce()

let abc = [99, 54, 123, 78, 82];
let average = abc.reduce((num, item, index) => {
    if(index != abc.length-1) {
        return num+item;
    } else {
        return (num+item)/abc.length;
    }
});
console.log('平均數：', average)