SOURCE

let arr = [
    { name: 'animal', id: 1, parent: null },
    { name: 'dog', id: 2, parent: 1 },
    { name: 'cat', id: 3, parent: 1 }
]


let obj = {
    id: 1, name: 'animal', children: [
        { id: 2, name: 'dog', children: null },
        { id: 3, name: 'cat', children: null }
    ]
}

let obj2 = arr.reduce(
    (result, item) => {
        
    }
, {})