数组的reduce()方法接受一个函数作为累加器，将数组的每个值从左到右开始用该函数进行处理，最终输出为一个值
最常见的用法就是数组求和：
let arr = [5, 4, 3, 2, 1];
let reducer = (total, currentValue, currentIndex, arr) => {
console.log('total', total);
console.log('currentValue', currentValue);
console.log('currentIndex', currentIndex);
console.log('arr', arr);
return total + currentValue
}
console.log(arr.reduce(reducer, 100))
console.log(arr.reduce(reducer))
console.log(arr)语法：
array.reduce(function(total, currentValue, currentIndex, arr), initialValue)其中参数：
- total：初始值或者计算后的返回值，必须
- currentValue：当前元素，必须
- currentIndex：当前元素索引，可选
- arr：当前数组，可选
- initialValue：传递给函数的舒初始值，可选
const array1 = [1, 2, 3, 4];
const reducer = (accumulator, currentValue) => accumulator + currentValue;
// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10
// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15