reduce 基本使用场景

const arr1 = [1, 2, 3, 4, 5]
const arr2 = [
    { name: '张三', age: 20, id: 1},
    { name: '李四', age: 20, id: 2},
    { name: '王五', age: 18, id: 3},
    { name: '赵六', age: 19, id: 4},
    { name: '钱七', age: 18, id: 5},
    { name: '孙八', age: 19, id: 6},
    { name: '李九', age: 20, id: 7}
]
// 场景一：number数组求和
const result1 = arr1.reduce((prev, curr) => prev + curr)

// 场景二：对象属性求和
const result2 = arr2.reduce((prev, curr) => prev + curr.age, 0)

// 场景三：对象根据age进行分组
const result3 = arr2.reduce((prev, curr) => {
    const index = prev.findIndex(v => v.age === curr.age)
    if (index >= 0) {
        prev[index].list.push(curr)
    } else {
        prev.push({
            age: curr.age,
            list: [curr]
        })
    }
    return prev
}, [])

// 场景四：数组转对象
const result4 = arr2.reduce((prev, curr) => {
    prev[curr.id] = curr
    return prev
}, {})