obgen

Obgen

Observable (reactive streams) pattern implemented using es2015 async generators.

Installation

Using yarn:

yarn add obgen

or using npm:

npm i --save obgen

Usage

Observables are lazy streams of data that emit items asynchronously. They may be infinite or include an optional terminal event to signal the end of the stream. You can map, filter, etc.:

import Observable from "obgen";
import {
  asyncDefer,
  buffer,
  empty,
  from,
  just,
  promise,
  wrap,
} from "obgen/factoryFunctions";

const arr = [...Array(num).keys()].map((_, i) => i);
const observable = from(arr)
  .map((i) => i * 2)
  .filter((i) => i % 2 == 0)
  .take(10);

Observables are lazily evaluated. Items are not collected until you subscribe to them:

observable.subscribe(console.log);
// outputs:
// 0
// 2
// 4
// 6
// 8
// 10
// 12
// 14
// 16
// 18

If you prefer, you can instead iterate it with for-await as you normally would:

for await (const element of observable.iterable()) {
  console.log(element);
}

Or collect the items into an array:

const array = await observable.toArray();

Observables can be created in multiple ways. For example, you can manually wrap an async generator function (which is not particularly useful by itself):

const observable = wrap(async function* () {
  yield "a";
  yield "b";
  yield "c";
});

You can also use buffer() to accumulate items until subscription time:

const observable = buffer((stream) => {
  stream.emit(1);
  stream.emit(2);
  stream.emit(3);
  stream.emit(4);
  stream.end();
});

Or asynchronously emit items:

const observable = buffer((stream) => {
  // delay emission for a few milliseconds so that it happens after we subscribe
  times(5, (i) => setTimeout(() => stream.emit(i), i * 100));
  setTimeout(() => stream.end(), 600);
});
expect(await observable.toArray()).toEqual([0, 1, 2, 3, 4]);

License

MIT