reduce函数语法：
array.reduce(function(total, currentValue, currentIndex, arr), initialValue)
total：必需。累积器，即上一次回调函数执行的返回值
currentValue：必需。当前元素
currentIndex：可选。当前元素的索引
arr：可选。当前元素所属的数组对象
initialValue：可选。传递给函数的初始值
使用场景案例
1、对数字累计求和
const arr= [1,23,4,5]
const sum = arr.reduce((total,current)=>{return current+ total})
2、统计数组中各个元素出现的次数
const fruits = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple'];
const count = fruits.reduce((accumulator, currentValue) => {accumulator[currentValue] = (accumulator[currentValue] || 0) + 1;return accumulator;
}, {});
console.log(count); // Output: { apple: 3, banana: 2, orange: 1 }3、求平均值
const grades = [85, 90, 92, 88, 95];
const average = grades.reduce((acc, curr, index, array) => {acc += curr;if (index === array.length - 1) {return acc / array.length;}return acc;
}, 0)
consloe.log(average ) //90
4、展平数组
const arr = [1, 2, 3, 4, [1, 2, 3, [1, 2, 3, [1, 2, 3]]], 5, "string", { name: "弹铁蛋同学" }]// 首先使用 reduce 展开一层
arr.reduce((pre, cur) => pre.concat(cur), []);
// [1, 2, 3, 4, 1, 2, 3, [1, 2, 3, [1, 2, 3]], 5, "string", { name: "弹铁蛋同学" }];// 用 reduce 展开一层 + 递归
const flat = arr => {return arr.reduce((pre, cur) => {return pre.concat(Array.isArray(cur) ? flat(cur) : cur);}, []);
};
// [1, 2, 3, 4, 1, 2, 3, 1, 2, 3, 1, 2, 3, 5, "string", { name: "弹铁蛋同学" }];
)
