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/RxJS

partition

function stable

Splits the source Observable into two, one with values that satisfy a predicate, and another with values that don't satisfy the predicate.

`partition<T>(source: ObservableInput<T>, predicate: (value: T, index: number) => boolean, thisArg?: any): [Observable<T>, Observable<T>]`

Parameters

source

Type: ObservableInput.

predicate

A function that evaluates each value emitted by the source Observable. If it returns true, the value is emitted on the first Observable in the returned array, if false the value is emitted on the second Observable in the array. The index parameter is the number i for the i-th source emission that has happened since the subscription, starting from the number 0.

thisArg

Optional. Default is undefined.

An optional argument to determine the value of this in the predicate function.

Returns

[Observable<T>, Observable<T>]: An array with two Observables: one with values that passed the predicate, and another with values that did not pass the predicate.

Description

It's like filter, but returns two Observables: one like the output of filter, and the other with values that did not pass the condition.

partition marble diagram

partition outputs an array with two Observables that partition the values from the source Observable through the given predicate function. The first Observable in that array emits source values for which the predicate argument returns true. The second Observable emits source values for which the predicate returns false. The first behaves like filter and the second behaves like filter with the predicate negated.

Example

Partition a set of numbers into odds and evens observables

import { of, partition } from 'rxjs';

const observableValues = of(1, 2, 3, 4, 5, 6);
const [evens$, odds$] = partition(observableValues, (value, index) => value % 2 === 0);

odds$.subscribe(x => console.log('odds', x));
evens$.subscribe(x => console.log('evens', x));

// Logs:
// odds 1
// odds 3
// odds 5
// evens 2
// evens 4
// evens 6