Reactive polyglot

R. (Ramda) stands for standard functional toolkit.

Comparison notes

Operator counterparts aren't and can't be fully equivalent.
"Kinda the same" – is the current definition.

The following projects, were created with different goals and tradeoffs in mind, so every comparison and analogy is subjective and can be argued. The presence of some operator is not necessary good, as well as the abscence is not necessary bad.

Note, that primitives differ for each library. In descriptions, we broadly refer to all the "observable primitives" as streams*, though, technically speaking, some of them are rather stream-like entities.

To find something, search for a term you know.

API

Create

Create an empty stream

• KefirJS: never
• MostJS: empty
• RxJS: of
• XStream: empty

Create a stream from a single value

• KefirJS: constant
• MostJS: of, just
• RxJS: of, just
• XStream: of

Create a stream from an array

• KefirJS: sequentially
• MostJS: from
• RxJS: from, of
• XStream: from, fromArray

Create a stream from a promise

• KefirJS: fromPromise
• MostJS: fromPromise
• RxJS: fromPromise
• XStream: fromPromise

Create a stream from an event

• KefirJS: fromEvents
• MostJS: fromEvent
• RxJS: fromEvent
• XStream: fromEvent

Create a stream from a callback

• KefirJS: stream, fromCallback, fromNodeCallback, fromPoll
• MostJS: new Stream
• RxJS: new Observable, bindCallback, bindNodeCallback
• XStream: ? (make a promise first)

Prepend a stream with a value

• KefirJS: merge + constant
• MostJS: startWith
• RxJS: startWith
• XStream: startWith

Transform (data events)

Map value one-to-one

• KefirJS: map
• MostJS: map, constant
• RxJS: map, mapTo
• XStream: map

Filter value by a predicate

• KefirJS: filter
• MostJS: filter
• RxJS: filter
• XStream: filter

Skip N initial values

• KefirJS: skip
• MostJS: skip
• RxJS: skip
• XStream: drop

Take N initial values

• KefirJS: take
• MostJS: take
• RxJS: take
• XStream: take

Make a value from an accumulator and a next value

• KefirJS: scan
• MostJS: scan
• RxJS: scan
• XStream: fold

Combine

Merge multiple streams together

• KefirJS: merge
• MostJS: merge
• RxJS: merge
• XStream: merge

Pay attention that Kefir's merge accepts arrays while others are variadic.

Combine multiple streams together

• KefirJS: combine
• MostJS: combine
• RxJS: combineLatest
• XStream: combine

Sample a stream by another stream

• KefirJS: combine, sampledBy
• MostJS: sample, sampleWith
• RxJS: sample, withLatestFrom
• XStream: sampleCombine

Chain streams sequentially

• KefirJS: flatMapConcat
• MostJS: concatMap
• RxJS: concatMap
• XStream: map + flattenSequentially

---

Create

Create stream from non-stream values.

KefirJS	MostJS	RxJS	XStream
interval	periodic	interval + map	periodic
repeat	of + R.range	repeat	of + R.range
?	iterate	generate	?
?	generate	generate	?

Mapper

Modify events one to one.

KefirJS	MostJS	RxJS	XStream
delay	delay	delay	combine(delay(500))
– (map)	timestamp	timestamp	– (map)

Transforms

Modify events * to *.

MostJS	RxJS	XStream
chain / flatMap	flatMap	map + flattenConcurrently
map + switch	switchMap / flatMapLatest	map + flatten
join	mergeAll	flatten
loop	scan + map	fold + map
– (custom)	bufferWithCount	?

Filters

Skip events by predicate or signal.

MostJS	RxJS	XStream
skipRepeats	distinctUntilChanged	dropRepeats
skipRepeatsWith	– (scan)	dropRepeats
slice	skip + take	drop + take
skipWhile	skipWhile	fold + filter + map
takeWhile	takeWhile	filter + endWhen
since / skipUntil	skipUntil	fold + filter + map
until / takeUntil	takeUntil	filter + endWhen
during	window + take(1)	?

Combinators

Combine multiple streams into single.

MostJS	RxJS	XStream
zip	zip	?
concat	concat	concat
ap	combineLatest	?

Side effects

Produce side effect for every event.

MostJS	RxJS	XStream
tap	do / tap	debug

Ending

Operators which target end event somehow.

MostJS	RxJS	XStream
empty	empty	empty
never	never	never
continueWith	?	concat

Concurrency

MostJS	RxJS
– (custom)	amb / race

History

MostJS	RxJS
– (custom)

Design diffs

RxJS

1. Three primitives: Observer, Observable, Subject.
2. Observables end on error.
3. Provides API to handle errors.
4. Does not provide API to handle ending.

KefirJS

1. Two primitives: Stream and Property (like XStream).
2. Observables does not end on error (by default).
3. Provides API to handle errors.
4. Provides API to handle ending.

MostJS

1. One primitive: Stream (+ community-driven).
2. Separate packages for subject-like and property-like primitives.
3. Provides API to handle errors.
4. Provides API to handle ending.

XStream

1. Two primitives: Stream and MemoryStream (like KefirJS).
2. Always multicast. A Stream is like an RxJS Subject.
3. Streams end on error.

Common

RxJS does not emit initial scan value as event (use startWith for that).

Rx.Observable.interval(100).map(x => 1)
  .scan(add, 0);
  .subscribe(console.log); // 1--2--3--...

Most.periodic(100, 1)
  .scan(add, 0);
  .observe(console.log); // 0--1--2--3--...

Found docs / API quirks

MostJS

startWith vs sampleWith vs continueWith + recoverWith vs skipRepeatsWith
(val vs none vs func vs stream)

tap is listed in "Transform" section.

RxJS

startWith is listed in "Combine" section.

mergeAll is listed in "Combine" section.

distinct is not listed in "Filtering" section.

takeUntil is not listed in "Filtering" section.

just / return should be deprecated in favor of of.

fromArray should be deprecated in favor of from.

Links

bacon-vs-kefir – BaconJS vs KefirJS API comparison

dataflows – web arch. dataflow comparison

stream-conversions – tool for cross-library stream conversions

Additional Read

https://github.com/cujojs/most/issues/171

https://twitter.com/rpominov/status/689566111734599683

https://github.com/zenparsing/es-observable/issues/66

Reactive polyglot

R. (Ramda) stands for standard functional toolkit.

Comparison notes

Operator counterparts aren't and can't be fully equivalent.
"Kinda the same" – is the current definition.

The following projects, were created with different goals and tradeoffs in mind, so every comparison and analogy is subjective and can be argued. The presence of some operator is not necessary good, as well as the abscence is not necessary bad.

Note, that primitives differ for each library. In descriptions, we broadly refer to all the "observable primitives" as streams*, though, technically speaking, some of them are rather stream-like entities.

To find something, search for a term you know.

API

Create

Create an empty stream

• KefirJS: never
• MostJS: empty
• RxJS: of
• XStream: empty

Create a stream from a single value

• KefirJS: constant
• MostJS: of, just
• RxJS: of, just
• XStream: of

Create a stream from an array

• KefirJS: sequentially
• MostJS: from
• RxJS: from, of
• XStream: from, fromArray

Create a stream from a promise

• KefirJS: fromPromise
• MostJS: fromPromise
• RxJS: fromPromise
• XStream: fromPromise

Create a stream from an event

• KefirJS: fromEvents
• MostJS: fromEvent
• RxJS: fromEvent
• XStream: fromEvent

Create a stream from a callback

• KefirJS: stream, fromCallback, fromNodeCallback, fromPoll
• MostJS: new Stream
• RxJS: new Observable, bindCallback, bindNodeCallback
• XStream: ? (make a promise first)

Prepend a stream with a value

• KefirJS: merge + constant
• MostJS: startWith
• RxJS: startWith
• XStream: startWith

Transform (data events)

Map value one-to-one

• KefirJS: map
• MostJS: map, constant
• RxJS: map, mapTo
• XStream: map

Filter value by a predicate

• KefirJS: filter
• MostJS: filter
• RxJS: filter
• XStream: filter

Skip N initial values

• KefirJS: skip
• MostJS: skip
• RxJS: skip
• XStream: drop

Take N initial values

• KefirJS: take
• MostJS: take
• RxJS: take
• XStream: take

Make a value from an accumulator and a next value

• KefirJS: scan
• MostJS: scan
• RxJS: scan
• XStream: fold

Combine

Merge multiple streams together

• KefirJS: merge
• MostJS: merge
• RxJS: merge
• XStream: merge

Pay attention that Kefir's merge accepts arrays while others are variadic.

Combine multiple streams together

• KefirJS: combine
• MostJS: combine
• RxJS: combineLatest
• XStream: combine

Sample a stream by another stream

• KefirJS: combine, sampledBy
• MostJS: sample, sampleWith
• RxJS: sample, withLatestFrom
• XStream: sampleCombine

Chain streams sequentially

• KefirJS: flatMapConcat
• MostJS: concatMap
• RxJS: concatMap
• XStream: map + flattenSequentially

---

Create

Create stream from non-stream values.

KefirJS	MostJS	RxJS	XStream
interval	periodic	interval + map	periodic
repeat	of + R.range	repeat	of + R.range
?	iterate	generate	?
?	generate	generate	?

Mapper

Modify events one to one.

KefirJS	MostJS	RxJS	XStream
delay	delay	delay	combine(delay(500))
– (map)	timestamp	timestamp	– (map)

Transforms

Modify events * to *.

MostJS	RxJS	XStream
chain / flatMap	flatMap	map + flattenConcurrently
map + switch	switchMap / flatMapLatest	map + flatten
join	mergeAll	flatten
loop	scan + map	fold + map
– (custom)	bufferWithCount	?

Filters

Skip events by predicate or signal.

MostJS	RxJS	XStream
skipRepeats	distinctUntilChanged	dropRepeats
skipRepeatsWith	– (scan)	dropRepeats
slice	skip + take	drop + take
skipWhile	skipWhile	fold + filter + map
takeWhile	takeWhile	filter + endWhen
since / skipUntil	skipUntil	fold + filter + map
until / takeUntil	takeUntil	filter + endWhen
during	window + take(1)	?

Combinators

Combine multiple streams into single.

MostJS	RxJS	XStream
zip	zip	?
concat	concat	concat
ap	combineLatest	?

Side effects

Produce side effect for every event.

MostJS	RxJS	XStream
tap	do / tap	debug

Ending

Operators which target end event somehow.

MostJS	RxJS	XStream
empty	empty	empty
never	never	never
continueWith	?	concat

Concurrency

MostJS	RxJS
– (custom)	amb / race

History

MostJS	RxJS
– (custom)

Design diffs

RxJS

1. Three primitives: Observer, Observable, Subject.
2. Observables end on error.
3. Provides API to handle errors.
4. Does not provide API to handle ending.

KefirJS

1. Two primitives: Stream and Property (like XStream).
2. Observables does not end on error (by default).
3. Provides API to handle errors.
4. Provides API to handle ending.

MostJS

1. One primitive: Stream (+ community-driven).
2. Separate packages for subject-like and property-like primitives.
3. Provides API to handle errors.
4. Provides API to handle ending.

XStream

1. Two primitives: Stream and MemoryStream (like KefirJS).
2. Always multicast. A Stream is like an RxJS Subject.
3. Streams end on error.

Common

RxJS does not emit initial scan value as event (use startWith for that).

Rx.Observable.interval(100).map(x => 1)
  .scan(add, 0);
  .subscribe(console.log); // 1--2--3--...

Most.periodic(100, 1)
  .scan(add, 0);
  .observe(console.log); // 0--1--2--3--...

Found docs / API quirks

MostJS

startWith vs sampleWith vs continueWith + recoverWith vs skipRepeatsWith
(val vs none vs func vs stream)

tap is listed in "Transform" section.

RxJS

startWith is listed in "Combine" section.

mergeAll is listed in "Combine" section.

distinct is not listed in "Filtering" section.

takeUntil is not listed in "Filtering" section.

just / return should be deprecated in favor of of.

fromArray should be deprecated in favor of from.

Links

bacon-vs-kefir – BaconJS vs KefirJS API comparison

dataflows – web arch. dataflow comparison

stream-conversions – tool for cross-library stream conversions

Additional Read

https://github.com/cujojs/most/issues/171

https://twitter.com/rpominov/status/689566111734599683

https://github.com/zenparsing/es-observable/issues/66