供參考

var new = Array.from(
  old.reduce((dict, item)=> {
    if (dict.has(item.name)) {
      dict.get(item.name).push(item.id)
    } else {
      dict.set(item.name, [item.id])
    }
    return dict
  }, new Map())
).map(item => ({a: item[1], b: item[0]}))

原數組是

var old = [
    {
        id: 1,
        name: "first"
    },
    {
        id: 2,
        name: "first"
    },
    {
        id: 3,
        name: "second"
    },
    {
        id: 4,
        name: "second"
    }
]

期望得到

new = [
    {
        a: [1, 2],
        b: "first"
    },
    {
        a: [3, 4],
        b: "second"
    }
]

實作

var getNew = old => {
    let temp = old.reduce((acc, cur) => {
        if (acc[cur.name]){
            acc[cur.name].push(cur.id); 
        } else {
            acc[cur.name] = [cur.id]
        }
        return acc; 
    }, {});
    
    return Object.keys(temp).map(key => {
        return {
            a: temp[key],
            b: key
        }
    })
}

Just Run getNew(old)