[](https://www.npmjs.com/package/@rimbu/stream)
# `@rimbu/stream`
**Lazy, composable streams for TypeScript & JavaScript.**
`@rimbu/stream` provides **Stream** and **AsyncStream** – efficient, iterable-like sequences
that can be finite or infinite, materialized (arrays, strings, collections) or computed
on the fly. Streams are **lazy** and **immutable**: you can chain many operations
(`map`, `filter`, `flatMap`, `window`, `groupBy`, …) without creating intermediate
collections until you finally consume the result.
Use streams when you want **fluent, high‑level data pipelines** that are still
performance-conscious and type-safe.
---
## Table of Contents
1. [Why `@rimbu/stream`?](#why-rimbu-stream)
2. [Feature Highlights](#feature-highlights)
3. [Quick Start](#quick-start)
4. [Core Concepts & Types](#core-concepts--types)
5. [Sync vs Async Streams](#sync-vs-async-streams)
6. [Performance Notes](#performance-notes)
7. [Installation](#installation)
8. [FAQ](#faq)
9. [Ecosystem & Integration](#ecosystem--integration)
10. [Contributing](#contributing)
11. [License](#license)
---
## Why `@rimbu/stream`?
Plain arrays and iterables are great, but they tend to push you toward:
- **Eager computation** – multiple passes and intermediate arrays for each step.
- **Manual control flow** – nested loops, counters, and early exits.
- **Ad‑hoc composition** – hard-to-reuse logic scattered across your code.
`@rimbu/stream` focuses on:
- **Laziness** – operations like `map`, `filter`, `flatMap`, `take`, `drop`, and `window`
are applied as you iterate; no intermediate arrays are allocated unless you ask for them.
- **Expressive APIs** – rich, chainable operators for both sync `Stream` and `AsyncStream`.
- **Interoperability** – work seamlessly with arrays, strings, iterables, async iterables,
and other Rimbu collections.
- **Immutable, persistent semantics** – derived structures and terminal results are safe
to share and reuse.
If you find yourself writing a lot of `for` loops over arrays or async sources,
Streams are usually a better, more declarative fit.
---
## Feature Highlights
- **Lazy & composable** – build rich pipelines without materializing intermediate values.
- **Sync and async variants** – `Stream` for synchronous sources, `AsyncStream` for async ones.
- **Flexible sources** – consume arrays, strings, iterables, async iterables, other streams,
and more via `StreamSource` / `AsyncStreamSource`.
- **Powerful operators** – mapping, filtering, windows, grouping, flattening, zipping,
splitting on separators or slices, and more.
- **Reducer / Transformer support** – reusable `Reducer` and `Transformer` utilities
for advanced aggregation and streaming transformations.
- **Type-safe non-empty variants** – `Stream.NonEmpty` and `AsyncStream.NonEmpty`
for stronger guarantees when you know a stream is non-empty.
Try Rimbu (including `@rimbu/stream`) live in the browser using the
[Rimbu Sandbox on CodeSandbox](https://codesandbox.io/s/github/vitoke/rimbu-sandbox/tree/main?previewwindow=console&view=split&editorsize=65&moduleview=1&module=/src/index.ts).
---
## Quick Start
### Basic `Stream` usage
```ts
import { Stream } from '@rimbu/stream';
// Create from arrays, strings, or other sources
const s = Stream.from([1, 2, 3, 4, 5]);
// Build a lazy pipeline
const result = s
.filter((v) => v % 2 === 1) // keep odd numbers
.map((v) => v * v) // square them
.take(3); // at most 3 values
console.log(result.toArray()); // [1, 9, 25]
```
### Ranged and infinite streams
```ts
import { Stream } from '@rimbu/stream';
// Finite range
console.log(Stream.range({ start: 10, amount: 5 }).toArray());
// => [10, 11, 12, 13, 14]
// Infinite stream: repeat pattern
const repeating = Stream.of('a', 'b').repeat(); // infinite
console.log(repeating.take(5).toArray());
// => ['a', 'b', 'a', 'b', 'a']
```
### `AsyncStream` usage
```ts
import { AsyncStream } from '@rimbu/stream/async';
async function* numbers() {
for (let i = 0; i < 5; i++) {
await new Promise((r) => setTimeout(r, 10));
yield i;
}
}
const s = AsyncStream.from(numbers());
const doubled = s.filter((v) => v % 2 === 0).map(async (v) => v * 2);
console.log(await doubled.toArray()); // [0, 4, 8]
```
---
## Core Concepts & Types
### Exported Types (sync)
From the main entry (`@rimbu/stream`):
| Name | Description |
| -------------------- | ------------------------------------------------------------------------------------------------------------------ |
| `FastIterator` | An iterator that extends the default `Iterator` with a `fastNext` method for more efficient iteration. |
| `FastIterable` | An `Iterable` that returns a `FastIterator` from its `[Symbol.iterator]` method. |
| `Stream` | A possibly infinite, lazy sequence of elements of type `T` with a rich set of transformation and aggregation APIs. |
| `Stream.NonEmpty` | A non-empty refinement of `Stream` with stronger type guarantees. |
| `Streamable` | An object that can create a `Stream` via `.stream()`. |
| `StreamSource` | A convenience type covering arrays, strings, iterables, streams and `Streamable` objects convertible to `Stream`. |
| `Reducer` | A stand‑alone, reusable reduction specification that turns a sequence of `I` into a single result `O`. |
| `Transformer` | A `Reducer` that produces `StreamSource` values, useful for reusable streaming transforms. |
### Exported Types (async)
From the async entry (`@rimbu/stream/async`):
| Name | Description |
| ------------------------- | ------------------------------------------------------------------------------------------------------------------- |
| `AsyncFastIterator` | An async iterator with `fastNext` for efficient asynchronous iteration. |
| `AsyncFastIterable` | An `AsyncIterable` that returns an `AsyncFastIterator`. |
| `AsyncStream` | A possibly infinite asynchronous sequence of elements, mirroring the `Stream` API. |
| `AsyncStream.NonEmpty` | A non-empty refinement of `AsyncStream`. |
| `AsyncStreamable` | An object that can create an `AsyncStream` via `.asyncStream()`. |
| `AsyncStreamSource` | Any source convertible to an `AsyncStream` (async streams, async iterables, sync streams, lazy factories, etc.). |
| `AsyncReducer` | An asynchronous reducer that can consume values of type `I` and eventually yield a result of type `O`. |
| `AsyncTransformer` | An `AsyncReducer` that produces `AsyncStreamSource` values, for reusable async streaming transformations. |
See the full [Stream docs](https://rimbu.org/docs/collections/stream) and
[API reference](https://rimbu.org/api/rimbu/stream) for all operations.
---
## Sync vs Async Streams
- **`Stream`** (sync) is ideal when your data source is already in memory or synchronously available:
arrays, strings, other collections, generated ranges, etc.
- **`AsyncStream`** is the async counterpart when values arrive over time:
async generators, network responses, file I/O, timers, etc.
Imports:
```ts
import { Stream } from '@rimbu/stream';
import { AsyncStream } from '@rimbu/stream/async';
```
Interoperability examples:
```ts
// Build an AsyncStream from a sync Stream
const sync = Stream.range({ start: 0, amount: 10 });
const asyncFromSync = AsyncStream.from(sync);
// Or consume async sources and materialize them synchronously later
async function load(): Promise {
return await AsyncStream.from(fetch('/api/data').then((r) => r.json()))
.map((item) => item.value)
.toArray();
}
```
Both APIs are intentionally similar so you can switch between sync and async
variants with minimal friction.
---
## Performance Notes
- Streams are built on **fast iterators** (`FastIterator` / `AsyncFastIterator`) to minimize
overhead in tight loops.
- Most operations are **lazy**: they only do work when you iterate, and they avoid
intermediate allocations unless you call methods like `toArray` or `groupBy`.
- Many terminal operations (like `reduce`, `fold`, `containsSlice`, `window`, `partition`,
`groupBy`) delegate to reusable `Reducer` / `AsyncReducer` definitions, making it easy
to share optimized aggregations across your codebase.
For detailed performance characteristics and benchmarks, see the main Rimbu
documentation at [rimbu.org](https://rimbu.org).
---
## Installation
### Node / Bun / npm / Yarn
```sh
npm install @rimbu/stream
# or
yarn add @rimbu/stream
# or
bun add @rimbu/stream
# or
deno add npm:@rimbu/stream
```
### Browser / ESM
`@rimbu/stream` ships both **ESM** and **CJS** builds. Use it with any modern bundler
(Vite, Webpack, esbuild, Bun, etc.) or directly in Node ESM projects.
---
## FAQ
**Q: How is `Stream` different from just using arrays?**
`Stream` is **lazy** and does not allocate intermediate arrays for each step. You can
express long pipelines and only materialize a result when you need it (e.g. via
`toArray`, `join`, `groupBy`, or other reducers).
**Q: Are streams immutable?**
Yes. Stream operations return **new stream instances**; the underlying sources are
not mutated. This makes them safe to reuse and share.
**Q: Can streams be infinite?**
Yes. Many constructors and operations (e.g. `Stream.range` without an `amount`,
`Stream.always`, `repeat`) can create infinite streams. Just make sure to apply
limiting operations like `take` when you need a finite result.
**Q: How do `Reducer` and `Transformer` relate to streams?**
`Reducer` and `AsyncReducer` encapsulate reusable reduction logic that can be applied
to any stream (e.g. `Reducer.sum`, `Reducer.minBy`, `Reducer.groupBy`).
`Transformer` and `AsyncTransformer` are reducers that output `StreamSource` /
`AsyncStreamSource` and power methods like `transform`, `window`, `splitOn`, and more.
---
## Ecosystem & Integration
- Part of the broader **Rimbu** ecosystem – interoperates with `@rimbu/core`,
`@rimbu/collection-types`, `@rimbu/hashed`, `@rimbu/ordered`, and others.
- Many Rimbu collections accept `StreamSource` / `AsyncStreamSource`, so you can
build data pipelines with streams and materialize them into maps, sets, lists, etc.
- Ideal for **data processing**, **ETL-like flows**, **reactive style pipelines**,
or anywhere you want a clear separation between _building_ and _consuming_ data.
Explore more at the [Rimbu documentation](https://rimbu.org) and the
[Stream API docs](https://rimbu.org/api/rimbu/stream).
---
## Contributing
We welcome contributions! See the
[Contributing guide](https://github.com/rimbu-org/rimbu/blob/main/CONTRIBUTING.md) for details.
_Made with [contributors-img](https://contrib.rocks)._
---
## License
MIT © Rimbu contributors. See [LICENSE](./LICENSE) for details.
---
## Attributions
Created and maintained by [Arvid Nicolaas](https://github.com/vitoke). Logo © Rimbu.