Sort Array Object in Array Object javascript

how to sort highest value in this data using javascript?

data = [{a: [{num:31}, {num:10}]},{a: [{num:4}, {num:9}]},{a: [{num:5}, {num:9}]}]

Expected

data = [{a: [{num:31}]},{a: [{num:9}]},{a: [{num:9}]}]

I try like this but never happen 🙂

const data_sort = data.sort((a, b) => {
                                let abc
                                if (a.a.length > 0) {
                                    abc = a.a.sort((x, y) => x.a - x.a);
                                }
                                return a - b
                            })

>Solution :

    let data = [{a: [{num:31}, {num:10}]},{a: [{num:4}, {num:9}]},{a: [{num:5}, {num:9}]}]
    data = data.map(item => ({a:[item.a.sort((a, b) => b.num-a.num)[0]]})).sort((a, b) => b.a[0].num-a.a[0].num)

console.log(data)