How can I merge these two Observables in Angular to create an array of objects?

Question

How can I merge these two Observables in Angular to create an array of objects?

Let's say we are working with two different datasets:

 student$ = from([
    {id: 1, name: "Alex"},
    {id: 2, name: "Marry"},
  ])

  address$ = from([
    {id: 1, location: "Chicago", sid: 1},
    {id: 2, location: "Florida", sid: 2},
  ])

The task at hand is to merge these two datasets into an array of objects. The desired output should resemble the following structure:

  studentAddress = [
    {
      id: 1,
      name: "Alex",
      address: [
        {id: 1, location: "Chicago", sid: 1},
      ]
    },
    {
      id: 2,
      name: "Marry",
      address: [
        {id: 2, location: "Florida", sid: 2},
      ]
    },
  ]

javascript angular typescript rxjs

Answer 1

Answer №1

If your students$ and address$ observable both emit arrays of objects.

  address$ = of([
    { id: 1, location: 'Chicago', sid: 1 },
    { id: 2, location: 'Florida', sid: 2 },
  ]);

  students$ = of([
    { id: 1, name: 'Alex' },
    { id: 2, name: 'Marry' },
  ]);

You can utilize the RxJs function combineLatest: This function combines multiple Observables to create an Observable whose values are calculated from the latest values of each input Observable.

combineLatest will wait to emit a value until all observables have emitted at least one value

combineLatest(
  [this.students$, this.address$],
  (students, address) =>
    students.map((s) => ({
      ...s,
      address: address.filter((a) => a.sid === s.id),
    })) // You can also use an optional projection function with combineLatest
).subscribe(console.log);

Required imports:

import { combineLatest, of } from 'rxjs';

Check out the Angular Demo showcasing the usage of combineLatest and forkJoin

Note: Instead of combibeLatest, you may consider using forkJoin in your scenario. forkJoin emits a value only when all observables have completed and then emits the last emitted value from each.

Answer 2

If your students$ and address$ observable both emit arrays of objects.

  address$ = of([
    { id: 1, location: 'Chicago', sid: 1 },
    { id: 2, location: 'Florida', sid: 2 },
  ]);

  students$ = of([
    { id: 1, name: 'Alex' },
    { id: 2, name: 'Marry' },
  ]);

You can utilize the RxJs function combineLatest: This function combines multiple Observables to create an Observable whose values are calculated from the latest values of each input Observable.

combineLatest will wait to emit a value until all observables have emitted at least one value

combineLatest(
  [this.students$, this.address$],
  (students, address) =>
    students.map((s) => ({
      ...s,
      address: address.filter((a) => a.sid === s.id),
    })) // You can also use an optional projection function with combineLatest
).subscribe(console.log);

Required imports:

import { combineLatest, of } from 'rxjs';

Check out the Angular Demo showcasing the usage of combineLatest and forkJoin

Note: Instead of combibeLatest, you may consider using forkJoin in your scenario. forkJoin emits a value only when all observables have completed and then emits the last emitted value from each.

Answer 3

Answer №2

To accomplish this task, consider utilizing the flatMap method from RxJs. This method allows you to pass the first value needed by the second one. Below is a code snippet example demonstrating the use of flatMap(), and also a link to an article showcasing other methods (the example below is extracted from the same article).

let first = Observable.of(10);
first.flatMap((operand1) => {
  return Observable.of(operand1 + 10);
 })
.subscribe(res => this.flatMappedStreams = {msg: '10 + 10 = ' + res});

For additional methods like concat, combine, forkJoin, you can refer to this article here

The key factor lies in understanding RxJs Operators. For further insight, consult their official documentation

Answer 4

To accomplish this task, consider utilizing the flatMap method from RxJs. This method allows you to pass the first value needed by the second one. Below is a code snippet example demonstrating the use of flatMap(), and also a link to an article showcasing other methods (the example below is extracted from the same article).

let first = Observable.of(10);
first.flatMap((operand1) => {
  return Observable.of(operand1 + 10);
 })
.subscribe(res => this.flatMappedStreams = {msg: '10 + 10 = ' + res});

For additional methods like concat, combine, forkJoin, you can refer to this article here

The key factor lies in understanding RxJs Operators. For further insight, consult their official documentation

Answer 5

Answer №3

Based on your observation about the arrays being retrieved from nested HTTP calls, it appears that you would benefit from

switchMap operator to transition from one HTTP call to another (other higher order mapping operators)
map operator for modifying data emitted by the observable
Array#map in conjunction with Array#filter for combining the arrays

var { of } = rxjs;
var { switchMap, map } = rxjs.operators;

const students$ = of([
  {id: 1, name: "Alex"},
  {id: 2, name: "Marry"},
]);

const addresses$ = of([
  {id: 1, location: "Chicago", sid: 1},
  {id: 2, location: "Florida", sid: 2},
]);

const result$ = students$.pipe(
  switchMap(students =>
    addresses$.pipe(
      map(addresses => 
        students.map(student => ({
          ...student,
          address: addresses.filter(address => address.id === student.id)
        }))
      )
    )
  )
);

result$.subscribe(console.log);

.as-console-wrapper { max-height: 100% !important; top: 0; }

<script src="https://unpkg.com/<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="e193998b92a1d7cfd5cfd1">[email protected]</a>/bundles/rxjs.umd.min.js"></script>

Answer 6

Based on your observation about the arrays being retrieved from nested HTTP calls, it appears that you would benefit from

switchMap operator to transition from one HTTP call to another (other higher order mapping operators)
map operator for modifying data emitted by the observable
Array#map in conjunction with Array#filter for combining the arrays

var { of } = rxjs;
var { switchMap, map } = rxjs.operators;

const students$ = of([
  {id: 1, name: "Alex"},
  {id: 2, name: "Marry"},
]);

const addresses$ = of([
  {id: 1, location: "Chicago", sid: 1},
  {id: 2, location: "Florida", sid: 2},
]);

const result$ = students$.pipe(
  switchMap(students =>
    addresses$.pipe(
      map(addresses => 
        students.map(student => ({
          ...student,
          address: addresses.filter(address => address.id === student.id)
        }))
      )
    )
  )
);

result$.subscribe(console.log);

.as-console-wrapper { max-height: 100% !important; top: 0; }

<script src="https://unpkg.com/<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="e193998b92a1d7cfd5cfd1">[email protected]</a>/bundles/rxjs.umd.min.js"></script>

How can I merge these two Observables in Angular to create an array of objects?

Answer №1

Answer №2

Answer №3

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