Selection and partitioning systems Creation Functions

Creation Functions select one Observable from multiple Observables or split one Observable into multiple Observables according to conditions.

What is Creation Functions for selection and splitting?

Creation Functions for selection and partitioning differ from those for combining, and have the following roles.

• Selection: Selects an Observable that satisfies specific conditions from among multiple Observables.
• Splitting: Splitting an Observable into multiple Observables according to conditions.

These operate in the opposite direction or from a different perspective than the "combine several into one" join.

Major Selection and Division Systems Creation Functions

Function	Description	Use Case
race	Adopt first published	Multiple data source competition
partition	Split in two by condition	Branching process for success/failure

Usage Criteria

race - select the fastest Observable

race` subscribes to multiple Observables at the same time, and adopts the first Observable that issues a value. Observables that are not subscribed will be automatically unsubscribed.

Use Case:.

• Adopt the fastest response from multiple API endpoints
• Timeout handling (original process vs. timer)
• Competition between cache and actual API calls

import { race, timer } from 'rxjs';
import { map } from 'rxjs';

// Adopt the fastest from multiple data sources
const fast$ = timer(1000).pipe(map(() => 'Fast API'));
const slow$ = timer(3000).pipe(map(() => 'Slow API'));

race(fast$, slow$).subscribe(console.log);
// Output: 'Fast API' (1The output is output after a second,slow$is canceled)

partition - partition by condition

partitionsplits an Observable into **two Observables** based on a conditional function. The return value is an array[if true, if false]`.

Use Case:.

• Separation of success and failure
• Separation of even and odd numbers
• Separation of valid and invalid data

import { of, partition } from 'rxjs';

const source$ = of(1, 2, 3, 4, 5, 6);

// Split into even and odd numbers
const [even$, odd$] = partition(source$, n => n % 2 === 0);

even$.subscribe(val => console.log('Even:', val));
// Output: Even: 2, Even: 4, Even: 6

odd$.subscribe(val => console.log('Odd:', val));
// Output: Odd: 1, Odd: 3, Odd: 5

Convert Cold to Hot

As shown in the table above, All Selection and Split System Creation Functions generate a Cold Observable. Each subscription initiates an independent execution.

You can convert a Cold Observable to a Hot Observable by using a multicast operator (share(), shareReplay(), etc.).

Practical example: sharing conflicting API requests

import { race, timer } from 'rxjs';
import { map, share } from 'rxjs';

// ❄️ Cold - Rerun competition for each subscription
const coldRace$ = race(
  timer(1000).pipe(map(() => 'Fast API')),
  timer(3000).pipe(map(() => 'Slow API'))
);

coldRace$.subscribe(val => console.log('Subscribers1:', val));
coldRace$.subscribe(val => console.log('Subscribers2:', val));
// → Each subscriber runs an independent competition (a2competition once)

// 🔥 Hot - Share competition results among subscribers
const hotRace$ = race(
  timer(1000).pipe(map(() => 'Fast API')),
  timer(3000).pipe(map(() => 'Slow API'))
).pipe(share());

hotRace$.subscribe(val => console.log('Subscribers1:', val));
hotRace$.subscribe(val => console.log('Subscribers2:', val));
// → 1Share the results of one competition

TIP

For more information, see [Basic Creation System - Cold to Hot Conversion](/en/guide/creation-functions/basic/#cold- to -hot-).

Correspondence with Pipeable Operator

Creation Functions for selection and division also have a corresponding Pipeable Operator.

Creation Function	Pipeable Operator
race(a$, b$)	a$.pipe(raceWith(b$))
partition(source$, predicate)	Cannot be used in a pipeline (Creation Function only)

NOTE

There is no Pipeable Operator version of partition. If you need a partition, use it as a Creation Function or split it manually using filter twice.

Next Steps

To learn more about how each Creation Function works and practical examples, click on the links from the table above.

Also, learn Creation Functions for Combinations and Creation Functions for Conditionals to understand the whole picture of Creation Functions.