RxJs groupBy with an array of objects each having key and values array

Question

So the requirement is like: I got a array of objects (e.g. Array)

[
   { empDept: 'Engineering', empLoc: 'Pune', empName: 'John' },
   { empDept: 'Engineering', empLoc: 'Mumbai', empName: 'Harry' },
   { empDept: 'HR', empLoc: 'Pune', empName: 'Denis' },
   { empDept: 'Finance', empLoc: 'Mumbai', empName: 'Elvis' },
]

I will have a function which will accept the name of the property as a parameter, on which the employee list will be grouped. So if the input parameter value is 'empDept', then I want the result as below:

[
   {
      key: 'Engineering',
      values: [
         { empDept: 'Engineering', empLoc: 'Pune', empName: 'John' },
         { empDept: 'Engineering', empLoc: 'Mumbai', empName: 'Harry' },
     ]
   },
   {
      key: 'HR',
      values: [
         { empDept: 'HR', empLoc: 'Pune', empName: 'Denis' },
     ]
   },
   {
      key: 'Finance',
      values: [
        { empDept: 'Finance', empLoc: 'Mumbai', empName: 'Elvis' },
     ]
   }
]

I'm using the RxJs groupBy operator to achieve this. I have tried the below code, but it doesn't seem to give the desired result:

this.groupedEmployees$ = employees$.pipe(
      groupBy(person => person.empDept),
      mergeMap(group => of({ key: group.key, values: group.pipe(mergeMap(x => toArray())) })),
    );

Appreciate your help.

Crysta

Does this answer your question? RXJS groupBy Observable <Object[]> – Eradis

Also: Most efficient method to groupby on an array of objects – Eradis

Answer 1

If you really want to stick with RxJS, you can use this piece of code:

import { from } from 'rxjs'; 
import { groupBy, mergeMap, reduce, toArray } from 'rxjs/operators';

const groupedEmployees$ = from([
  { empDept: 'Engineering', empLoc: 'Pune', empName: 'John' },
  { empDept: 'Engineering', empLoc: 'Mumbai', empName: 'Harry' },
  { empDept: 'HR', empLoc: 'Pune', empName: 'Denis' },
  { empDept: 'Finance', empLoc: 'Mumbai', empName: 'Elvis' }
]).pipe(
  groupBy(person => person.empDept),
  mergeMap(group => group
    .pipe(
      reduce((acc, cur) => {
          acc.values.push(cur);
          return acc;
        },
        { key: group.key, values: [] }
      )
    )
  ),
  toArray()
);

groupedEmployees$.subscribe(console.log);

Wieren

Thanks, but why does it console.log twice, separately for each key-value pairs. I fixed it by adding a toArray() operator. – Crysta

I'm sorry, I didn't notice you wanted an array of these objects. Yes, adding toArray() would convert it to what you wanted. I'll edit my question. – Wieren

Answer 2

Why would you use rxjs in this case? It's purely array transformation. You can use lodash groupBy: https://lodash.com/docs/#groupBy

const t = [
   { empDept: 'Engineering', empLoc: 'Pune', empName: 'John' },
   { empDept: 'Engineering', empLoc: 'Mumbai', empName: 'Harry' },
   { empDept: 'HR', empLoc: 'Pune', empName: 'Denis' },
   { empDept: 'Finance', empLoc: 'Mumbai', empName: 'Elvis' },
];

const result = Object.entries(_.groupBy(t, 'empDept')).map(([key, values]) => ({key, values}));

Cowry

Thanks for you answer. But I prefer RxJs because I'm dealing with observables. – Crysta

That's what we call overkill: you make it more complex than it should be. Do not forget rxjs purpose: RxJS is a library for composing asynchronous and event-based programs by using observable sequences. It provides one core type, the Observable, satellite types (Observer, Schedulers, Subjects) and operators inspired by Array#extras (map, filter, reduce, every, etc) to allow handling asynchronous events as collections. and Think of RxJS as Lodash for events. – Cowry

Why would using RxJS be considered an overkill whilst using Lodash wouldn't? – Wieren

You can even do it without Lodash. With lodash, you can write it in 1 line like I did, with only one method groupBy. How many lines did you write with rxjs, and with how many functions? – Cowry

Yes, Lodash fixes this problem in one line, but what if OP's employees$ Observable is an asynchronous source that was already handled by RxJS so they just wanted to continue using RxJS not wanting to mix different approaches (e.g. adding Lodash solution)? I would call using two libraries as an overkill. And I'm not sure how would you solve this in one line without Lodash. – Wieren

Mixing libs is not a problem if they are modular. In case of lodash, use lodash-es. If you want to transform an Observable, there is map operator. – Cowry

Answer 3

Are you looking for something like this?

function groupBy(key, array) {
  return array.reduce((all, current) => {
    const existingKey = all.find(existing => existing.key === current[key]);
    if (!existingKey) {
      all.push({key: current[key], values: [current]});
    } else {
      existingKey.values.push(current);
    }
    return all;
  }, []);
}

Example:

const arr = [
   { empDept: 'Engineering', empLoc: 'Pune', empName: 'John' },
   { empDept: 'Engineering', empLoc: 'Mumbai', empName: 'Harry' },
   { empDept: 'HR', empLoc: 'Pune', empName: 'Denis' },
   { empDept: 'Finance', empLoc: 'Mumbai', empName: 'Elvis' },
];

const byDept = groupBy('empDept', arr);
const byLoc = groupBy('empLoc', arr);

Aedile