JS 数组怎么提取相同元素对应的值

Question

arr1通过字段a，取字段b，组装成arr2

arr1 = [
    {
        a:'8:00'
        b:1
    },
    {
        a:'8:00'
        b:2
    },
    {
        a:'8:00'
        b:3
    },
    {
        a:'8:00'
        b:4
    },
    {
        a:'9:00'
        b:1
    },
    {
        a:'9:00'
        b:2
    },
    {
        a:'9:00'
        b:3
    },                        
]
arr2: [
    {
        items: ['8:00', 1,2,3,4],
    },
    {
        items: ['9:00', 1,2,3],
    },
],  

Answer 1

arr1.reduce((arr, item) => {
    let res = arr.find(ci => ci.items.indexOf(item.a) == 0)
    if(res){
        res.items.push(item.b)
    }else{
        arr.push({items:[item.a, item.b]})
    }
    return arr
}, [])