Utilizing mapping is an effective strategy

var arr=[
  {"id":1,"symbol":"SP-500","date":"1927-12-30T07:00:00.000+00:00","open":17.66,"high":17.66,"low":17.66,"close":17.66,"volume":0},
  {"id":2,"symbol":"SP-500","date":"1928-01-03T07:00:00.000+00:00","open":17.76,"high":17.76,"low":17.76,"close":17.76,"volume":0}
]



Object.keys(arr[0]).forEach(el=>{
  let arrray= (el=="date") ? arr.map(ele=> ele[el].substring(0,10)) : arr.map(ele=>ele[el]);
  console.log(el,':', arrray);
  // implement your custom logic here
})

Subscribe for updates

return this.http.get(`${this.baseUrl}`).subscribe(arr=>{
   Object.keys(arr[0]).forEach(el=>{
      let arrray= (el=="date") ? arr.map(ele=> ele[el].substring(0,10)) : 
      arr.map(ele=>ele[el]);
      console.log(el,':', arrray);
      // implement your specific logic here
   })
});

The correct way to handle the resulting array from the subscribe method is crucial. Inside the callback function of subscribe, you will receive an actual array, allowing you to use your original approach with the .map method that was ineffective on the observer object in your first attempt.

fetchQuotes(): Observable<any> {
  let quotes = this.http.get<Quote[]>(`${this.baseUrl}`);
  quotes.subscribe((arr: Quote[]) => {
    const opens = arr.map(quote => quote.open)
    console.log(opens); // [17.66,17.76]

    const dates = arr.map(quote => quote.date.split("T")[0])
    console.log(dates); // [1927-12-30,1928-01-03]
  )}
  return quotes;
}

This code has a distinct odor... I firmly believe that the QuoteService method titled "getQuotesList" should have a singular purpose: to return an Observable just like you initially did.

In your component, I suggest storing the quote observable and creating a new observable using a switchMap operator with reduce logic inside.

For a potential solution, please refer to this stackblitz link: https://stackblitz.com/edit/rxjs-uurnuv?file=index.ts

Cheers!

An issue has been identified in the getQuotesList() function.

import { of, Observable } from 'rxjs'; 
import { map } from 'rxjs/operators';

const data = [
  {"id":1,"symbol":"SP-500","date":"1927-12-30T07:00:00.000+00:00","open":17.66,"high":17.66,"low":17.66,"close":17.66,"volume":0},
  {"id":2,"symbol":"SP-500","date":"1928-01-03T07:00:00.000+00:00","open":17.76,"high":17.76,"low":17.76,"close":17.76,"volume":0}
];

function getQuotesList(): Observable<[string[], string[]]> {
  // update to fetch data from this.http.get(this.baseUrl) instead of using of(data)
  return of(data).pipe(
    map((quotes) => {
      return quotes.reduce((acc, curr) => {
        acc[0].push(curr.date.substr(0, 10));
        acc[1].push(curr.open);
        return acc;
      }, [[], []]);
    }),
  )
}

getQuotesList().subscribe(x => console.log(x));

https://stackblitz.com/edit/rxjs-j8ptal?file=index.ts

UPDATE:

The complete class definition for your service is as follows:

import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { Observable, Subject, of } from "rxjs";
import { map } from 'rxjs/operators';
import "rxjs/add/operator/map";
import "rxjs/operator/filter";
import { Quote } from "./quote";

@Injectable({
  providedIn: "root"
})
export class QuoteService {
  private baseUrl = "http://localhost:8080/springboot-crud-rest/api/v1/quotes";

  constructor(private http: HttpClient) {}

  getQuote(id: number): Observable<any> {
    return this.http.get(`${this.baseUrl}/${id}`);
  }

  createQuote(quote: Object): Observable<Object> {
    return this.http.post(`${this.baseUrl}`, quote);
  }

  updateQuote(id: number, value: any): Observable<Object> {
    return this.http.put(`${this.baseUrl}/${id}`, value);
  }

  deleteQuote(id: number): Observable<any> {
    return this.http.delete(`${this.baseUrl}/${id}`, { responseType: "text" });
  }

  getQuotesList(): Observable<[string[], string[]]> {
    // replace of(data) with this.http.get(this.baseUrl)
    return this.http.get<Quote[]>(this.baseUrl).pipe(
      map(quotes => {
        return quotes.reduce(
          (acc, curr) => {
            acc[0].push(curr.date.substr(0, 10));
            acc[1].push(curr.open);
            return acc;
          },
          [[], []]
        );
      })
    );
  }
}

I strive to comprehend your desires.

1. Your aim is to dissect json data into structured data of a specific type.
2. You intend to consolidate data into array values.

Is that correct?

Allow me to provide you with some code.

interface Quote {
  id: number;
  symbol: string;
  date: string;
  open: number;
  high: number;
  low: number;
  close: number;
  volume: number;
}

const jsonList = [{
    "id": 1,
    "symbol": "SP-500",
    "date": "1927-12-30T07:00:00.000+00:00",
    "open": 17.66,
    "high": 17.66,
    "low": 17.66,
    "close": 17.66,
    "volume": 0
  },
  {
    "id": 2,
    "symbol": "SP-600",
    "date": "1928-01-03T07:00:00.000+00:00",
    "open": 17.76,
    "high": 17.76,
    "low": 17.76,
    "close": 17.76,
    "volume": 0
  }
]


function jsonParse < T = any > (json: any): T | null {
  try {
    return JSON.parse(JSON.stringify(json));
  } catch (error) {
    console.error('ERROR')
    return null;
  }
}

function aggregate < T = any > (dataList: T[]) {
  const aggregation = dataList.reduce < T[keyof T][][] > ((acc, data) => {
    Object.values(data).forEach((value: T[keyof T], index: number) => {
      if (Array.isArray(acc[index])) {
        acc[index].push(value);
        return acc;
      }
      acc[index] = [value]
      return acc;
    })
    return acc;
  }, []);

  return aggregation;
}

const quotes = jsonParse < Quote[] > (jsonList);
if (quotes) {
  const aggregationQuotes = aggregate < Quote > (quotes);
  console.log('quotes', quotes)
  console.log('aggregationQuotes', aggregationQuotes)
}