@johngw/stream/dist/transformers/withLatestFrom.js

Reactive programming tools using the WHATWG Streams API.

import { empty } from '@johngw/stream-common/Symbol';
import { ControllableSource } from '../sources/ControllableSource.js';
import { SourceComposite } from '../sources/SourceComposite.js';
/**
 * Combines the source Observable with other Observables to create an Observable
 * whose values are calculated from the latest values of each, only when the source emits.
 *
 * @group Transformers
 * @example
 * ```
 * --a-----b-------c---d---e--
 *
 * withLatestFrom(
 * -----1----2-3-4------------
 * )
 *
 * --------b1------c4--d4--e4-
 * ```
 */
export function withLatestFrom(...inputs) {
    const abortController = new AbortController();
    const isFilled = (arr) => arr.every((value) => value !== empty);
    const inputValues = inputs.map(() => empty);
    inputs.forEach((input, index) => input
        .pipeTo(new WritableStream({
        abort(reason) {
            abortController.abort(reason);
        },
        write(chunk) {
            inputValues[index] = chunk;
        },
    }), { signal: abortController.signal })
        .catch(() => { }));
    const controller = new ControllableSource();
    const readable = new ReadableStream(new SourceComposite([
        controller,
        {
            start,
            cancel(reason) {
                abortController.abort(reason);
            },
        },
    ]));
    const writable = new WritableStream({
        start,
        write(chunk) {
            if (isFilled(inputValues))
                controller.enqueue([chunk, ...inputValues]);
        },
        abort(reason) {
            abortController.abort(reason);
        },
        close() {
            controller.close();
        },
    });
    return { readable, writable };
    function start(controller) {
        const onAbort = () => controller.error(abortController.signal.reason);
        abortController.signal.addEventListener('abort', onAbort);
    }
}
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