Yu need to iterate over the array, filter out the items that have the matching the parent id and sum the counts using reduce() and add that to the existing count on each item.

const arr = [
  { id: 1, count: 0, parentId: null },
  { id: 2, count: 30, parentId: 1 },
  { id: 3, count: 1, parentId: 2 }
];

const result= [];
arr.forEach((item)=> {
  const  id = item.id;
  const items = arr.filter(x => x.parentId === item.id);
  const totalCount = item.count +  items.reduce((partialSum, a) => partialSum + a.count, 0);
  result.push({id,totalCount})
})

 console.log(result); // gives the following
 /*[
  {id: 1, totalCount:30}, 
  {id: 2, totalCount:31},
  {id: 3, totalCount: 1} 
]*/

this code will give you the desired result:

const arr = [
    { id: 1, count: 0, parentId: null },
    { id: 2, count: 30, parentId: 1 },
    { id: 3, count: 1, parentId: 2 }
];
const newArr = arr.map((element) => {
    const parent = arr.find((item) => {
        return item.parentId === element.id
    });
    const totalCount = parent ? element.count + parent.count : element.count;
    return {id: element.id, totalCount};
});
console.log(newArr);