How to pipe two ReadableStreams into one WritableStream in Javascript?

I have two ReadableStreams, and I want to pipe them into one WritableStream, where any data that comes through the ReadableStreams goes directly into the WritableStream right then.

I can do the opposite, by using ReadableStream.prototype.tee() to split one ReadableStream into two, but I do not know how to combine two into one.

const textarea = document.querySelector("textarea");


// This is a ReadableStream which says "Mom! " every 1 second.
const momReadableStream = new ReadableStream({ start: controller => {
  const sayMom = () => controller.enqueue("Mom! ");
  setInterval(sayMom, 1000);
}});

// This is a ReadableStream which says "Lois! " every 0.7 seconds.
const loisReadableStream = new ReadableStream({ start: controller => {
  const sayLois = () => controller.enqueue("Lois! ");
  setInterval(sayLois, 700);
}});

// This is a WritableStream which displays what it receives in a textarea.
const writableStream = new WritableStream({ write: (chunk, controller) => textarea.value += chunk });


momReadableStream.pipeTo(writableStream).catch(console.error); // Works fine, words display
loisReadableStream.pipeTo(writableStream).catch(console.error); // Words do not display, and Errors with "Failed to execute 'pipeTo' on 'ReadableStream': Cannot pipe to a locked stream"

<textarea readonly></textarea>

Answer

Manually, by racing the most recent read from each reader to produce the overall read and initiating those reads as necessary:

const never = new Promise(() => {});

const mergeStreams = streams => {
    const readers = streams.map(s => s.getReader());
    const reads = streams.map(() => null);
    const dones = [];
    const allDone = Promise.all(streams.map(s => new Promise(resolve => {
        dones.push(resolve);
    })));

    return new ReadableStream({
        start: controller => {
            allDone.then(() => {
                controller.close();
            });
        },
        pull: controller =>
            Promise.race(
                readers.map((r, i) =>
                    reads[i] ??= r.read().then(({value, done}) => {
                        if (done) {
                            dones[i]();
                            return never;
                        }

                        controller.enqueue(value);
                        reads[i] = null;
                    })
                )
            ),
        cancel: reason => {
            for (const reader of readers) {
                reader.cancel(reason);
            }
        },
    });
};

const textarea = document.querySelector("textarea");


const never = new Promise(() => {});

const mergeStreams = streams => {
    const readers = streams.map(s => s.getReader());
    const reads = streams.map(() => null);
    const dones = [];
    const allDone = Promise.all(streams.map(s => new Promise(resolve => {
        dones.push(resolve);
    })));

    return new ReadableStream({
        start: controller => {
            allDone.then(() => {
                controller.close();
            });
        },
        pull: controller =>
            Promise.race(
                readers.map((r, i) =>
                    reads[i] ??= r.read().then(({value, done}) => {
                        if (done) {
                            dones[i]();
                            return never;
                        }

                        controller.enqueue(value);
                        reads[i] = null;
                    })
                )
            ),
        cancel: reason => {
            for (const reader of readers) {
                reader.cancel(reason);
            }
        },
    });
};


// This is a ReadableStream which says "Mom! " every 1 second.
const momReadableStream = new ReadableStream({ start: controller => {
  const sayMom = () => controller.enqueue("Mom! ");
  setInterval(sayMom, 1000);
}});

// This is a ReadableStream which says "Lois! " every 0.7 seconds.
const loisReadableStream = new ReadableStream({ start: controller => {
  const sayLois = () => controller.enqueue("Lois! ");
  setInterval(sayLois, 700);
}});

// This is a WritableStream which displays what it receives in a textarea.
const writableStream = new WritableStream({ write: (chunk, controller) => textarea.value += chunk });


mergeStreams([
  momReadableStream,
  loisReadableStream,
]).pipeTo(writableStream).catch(console.error);

<textarea readonly></textarea>