Typed group by in JavaScript/TypeScript - nested JSON to a typed nested array

Question

I have this list in my front-end typescript file:

poMonths:

0: {id: 1, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2019-12-15T00:00:00", purchaseMonthString: "Dec-2019" , year: 2019, month: "December"}
1: {id: 2, companyName: "company5", companyId: 5, flActive: true, purchaseMonth: "2019-12-15T00:00:00", …}
2: {id: 3, companyName: "company13", companyId: 13, flActive: true, purchaseMonth: "2019-11-15T00:00:00", …}
3: {id: 4, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2019-11-15T00:00:00", …}
4: {id: 5, companyName: "company5", companyId: 5, flActive: true, purchaseMonth: "2019-10-15T00:00:00", …}
5: {id: 6, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2020-09-15T00:00:00", …}
6: {id: 7, companyName: "company7", companyId: 7, flActive: true, purchaseMonth: "2020-09-15T00:00:00", …}

I would like to get a nested typed tree out of it, something similar to this but in a typed format:

So I am using this function to group the list to what I want:

groupBy() {
    const result = this.poMonths.reduce((state, current) => {
      const { companyName, year, month } = current;

      const company = state[companyName] || (state[companyName] = {});
      const yearObj = company[year] || (company[year] = {});
      const monthArr = yearObj[month] || (yearObj[month] = []);

      monthArr.push(current);


      return state;
    }, {});
    return result;
  }

However, the returned value is not typed, it is just a JSON object. How can I make it typed utilizing these types for instance?:

export class ItemNode {
  children: ItemNode[];
  item: string;
}

/** leaf item node with database id information. each leaf will be a single leaf containing an id from the database */
export class LeafItemNode {
  id?: number;
  companyId: number;
  companyName: string;
  flActive: boolean;
  purchaseMonth: Date;
  purchaseMonthString: string;
  year: number;
  month: number;
}

Basically, the tree should consist of ItemNodes all the way down to the leaves which would be LeafItemNode (containing the IDs)

Answer 1

You can keep using .reduce() however now you have to start with an array as inital object:

let initialState: ItemNode[] = [];

const result: ItemNode[] = input.reduce((state, current) => {
    const { companyName, year, month } = current;

        let company = state.find(x => x.item == companyName);
        if (!company) {
            company = new ItemNode();
            company.item = companyName;
            company.children = [];
            state.push(company);    
        }

        let yearObj = company.children.find(x => x.item == year.toString());
        if (!yearObj) {
            yearObj = new ItemNode();
            yearObj.item = year.toString();
            yearObj.children = [];
            company.children.push(yearObj);               
        } 

        let monthObj = yearObj.children.find(x => x.item == month.toString());
        if (!monthObj) {
            monthObj = new ItemNode();
            monthObj.item = month.toString();
            monthObj.children = [];
            yearObj.children.push(monthObj);
        }

        let cur = new ItemNode();
        cur.item = id.toString();
        monthObj.children.push(cur);

        return state;
}, initialState);

JS testable version:

 class ItemNode {} let input = [{id: 1, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2019-12-15T00:00:00", year: 2019, month: "December"}, {id: 2, companyName: "company5", companyId: 5, flActive: true, purchaseMonth: "2019-12-15T00:00:00", year: 2019, month: "December"}, {id: 3, companyName: "company13", companyId: 13, flActive: true, purchaseMonth: "2019-11-15T00:00:00", year: 2019, month: "November"}, {id: 4, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2019-11-15T00:00:00", year: 2019, month: "December"}, {id: 5, companyName: "company5", companyId: 5, flActive: true, purchaseMonth: "2019-10-15T00:00:00", year: 2019, month: "October"}, {id: 6, companyName: "company14", companyId: 14, flActive: true, purchaseMonth: "2020-09-15T00:00:00", year: 2020, month: "September"}, { id: 7, companyName: "company7", companyId: 7, flActive: true, purchaseMonth: "2020-09-15T00:00:00", year: 2020, month: "September" }] let initialState = []; const result = input.reduce((state, current) => { const { id, companyName, year, month } = current; let company = state.find(x => x.item == companyName); if (!company) { company = new ItemNode(); company.item = companyName; company.children = []; state.push(company); } let yearObj = company.children.find(x => x.item == year.toString()); if (!yearObj) { yearObj = new ItemNode(); yearObj.item = year.toString(); yearObj.children = []; company.children.push(yearObj); } let monthObj = yearObj.children.find(x => x.item == month.toString()); if (!monthObj) { monthObj = new ItemNode(); monthObj.item = month.toString(); monthObj.children = []; yearObj.children.push(monthObj); } let cur = new ItemNode(); cur.item = id.toString(); monthObj.children.push(cur); return state; }, initialState); console.log(result);