rxjs if-then-else operator, shorthand for partition/merge

License: Apache-2.0

Language: TypeScript



rxjs 'if-then-else' operator, shorthand for partition/merge, for more fluid, slightly less cluttered code:

import when from 'rx-when' // default export can be renamed to 'ifThenElse'...
import { interval } from 'rxjs/observable/interval'
import { map, take } from 'rxjs/operators'

  when(v => !(v % 2))(
    map(v => `${v} is even`), // 'then' operator
    map(v => `${v} is odd`) // 'else' operator, optional
.subscribe(debug('next:'), debug('error:'), debug('done'))

instead of:

import { merge } from 'rxjs/observable/merge'
import { interval } from 'rxjs/observable/interval'
import { map, partition, take } from 'rxjs/operators'

const source$ = interval(1000).pipe(take(5))
const [ even$, odd$ ] = partition(v => !(v % 2))(source$)
  map(v => `${v} is even`)(even$),
  map(v => `${v} is odd`)(odd$)
.subscribe(debug('next:'), debug('error:'), debug('done'))

see the above example in this directory. run the example in your browser locally with npm run example or online here.


declare function when<I, O>(predicate: (v: I) => boolean): (
  onTrue: RxOperator<I, O>,
  onFalse?: RxOperator<I, O> // default to identity function
) => RxOperator<I, O>

type RxOperator<I,O> = ($: Observable<I>) => Observable<O>

the when operator splits the input stream according to a predicate, processes the partitioned streams with the corresponding operators (onTrue and onFalse), and merges both resulting streams back into a single output stream.

the onFalse operator is optional, defaults to the identity function that leaves the stream as is.

note that since the when operator is this module's default export, it can easily be renamed to anything else, e.g. ifThenElse.


although this library is written in TypeScript, it may also be imported into plain JavaScript code: modern code editors will still benefit from the available type definition, e.g. for helpful code completion.


Copyright 2018 Stéphane M. Catala

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and Limitations under the License.

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Stephane Catala

Packages Referencing this Repo

rxjs if-then-else operator, shorthand for partition/merge
Latest release 1.0.1 - Updated

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v1.0.1 May 05, 2018
v1.0.0 February 23, 2018

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