code.ts

const toChunks = (input: string, chunkSize) => {
    const i = Math.ceil(Math.random() * Math.min(chunkSize, input.length))

    if (!input.length) {
        return []
    }

    const head = input.slice(0, i)
    const tail = input.slice(i, input.length)

    return [head, ...toChunks(tail, chunkSize)]
}

const chunks = toChunks('hello world foo bar baz 1234567!!!11', 5)


const bufferToSize = (chunkSize) => (source) =>
    Observable.create(subscriber => {
        let buffer = ''

        return source.subscribe({
            next: (value) => {
                buffer += value

                while (buffer.length > chunkSize) {
                    subscriber.next(buffer.slice(0, chunkSize))
                    buffer = buffer.slice(chunkSize, buffer.length)
                }
            },
            complete: () => {
                subscriber.next(buffer)
                subscriber.complete()
            }
        })
    });


from(chunks)
    .pipe(bufferToSize(4))
    .subscribe((v: string) => console.log(`value: '${v}' :: len: ${v.length}`))

from(chunks)
    .pipe(
        Rx.concatMap((v: string) => from(v)),
        Rx.bufferCount(4),
        Rx.map(v => v.join(''))
    )
    .subscribe((v: string) => console.log(`value: '${v}' :: len: ${v.length}`))