Convert elements to array

convertToArray.js

const arr = [

    {

        id: 10,

        title: "Test 10",

        item: "Item 1"

    },

    {

        id: 10,

        title: "Test 10",

        item: "Item 2"

    },
    {

        id: 10,

        title: "Test 10",

        item: "Item 2"

    },

    {

        id: 11,

        title: "Test 11",

        item: "Item 1"

    },

    {

        id: 11,

        title: "Test 11",

        item: "Item 2"

    },

    {

        id: 11,

        title: "Test 11",

        item: "Item 3"

    },

    {

        id: 11,

        title: "Test 11",

        item: "Item 3"

    },

    {

        id: 11,

        title: "Test 11",

        item: "Item 4"

    },

]

const flatten = (acc, el) => {
    const element = acc.find(f => f.id === el.id)

    element
        ?  element.itens = [...new Set(element.itens.concat(el.item))] 
        :  acc.push({
            id: el.id,
            title: el.title,
            itens: [el.item]
        })

    return acc
}

const convertArr = arr => arr.reduce((flatten), [])

console.log(convertArr(arr))