Proposal: `yieldSame` for async generators

[aralroca]

When we do HTML streaming, currently, async generators are the last ones to control the chunks better and render streaming lists incrementally (see example). But if we want to show a title with the number of elements rendered in the list, which is updated in streaming, it is currently impossible to do easily.

import { Database } from "bun:sqlite";

const db = new Database("db.sqlite");

export default function MovieList() {
  return (
   <>
      <h2>Number of movies: {/* ??? */}</h2>
      <ul>
        <MovieItems />
      </ul>
   </>
  );
}

// Streaming HTML from SQLite query
async function* MovieItems() {
  for (const movie of db.query("SELECT title, year FROM movies")) {
    yield (
      <li>
        {movie.title} ({movie.year})
      </li>
    );
  }
}

Proposal: Add the yieldSame API to allow for the modification of the same chunk each time instead of increasing the number of chunks.

Example:

import { Database } from "bun:sqlite";

const db = new Database("db.sqlite");

export default function MovieList() {
  const movies = db.query("SELECT title, year FROM movies");

  return (
   <>
      <Title movies={movies} />
      <ul>
        <MovieItems movies={movies} />
      </ul>
   </>
  );
}

async function* Title({ movies }) {
  let i = 0;

  for (const movie of movies) {
    yield (
      <h2>
        Number of movies {++i}
      </h2>
    );
  }
}

// Modifying same chunk in Streaming HTML
Title.yieldSame = true;

// Streaming HTML from SQLite query
async function* MovieItems({ movies }) {
  for (const movie of movies) {
    yield (
      <li>
        {movie.title} ({movie.year})
      </li>
    );
  }
}