fast-merge-async-iterators: merge AsyncIterables with all corner cases covered

The idea is to build a ES2018+ compatible module which really covers all the features of AsyncIterator, AsyncIterable, AsyncGenerator and doesn't throw the baby out with the bathwater.

async function* gen1() { ... yield ... }
async function* gen2() { ... yield ... }
async function* gen3() { ... yield ... }
...
for await (merge(gen1(), gen2(), gen3())) { ... }
...
for await (merge("iters-close-wait", gen1(), gen2(), gen3())) { ... }

• Interleaves the values yielded by the inner AsyncIterables as soon as they arrive.
• Supports exceptions propagation down the stack: if an inner iterator throws, then all other iterators will be closed (with or without waiting), and then the exception will be delivered to the caller.
• Works fast and with no memory leak in Promise.race().
• Closes merging iterators correctly once the caller stops iterating the merged iterator: calls .return() for them which effectively triggers all their finally {} blocks.

Usage Example

import merge from "fast-merge-async-iterators";

async function* generator(name: string, dt: number) {
  try {
    for (let i = 0; ; i++) {
      console.log(`${name} yielded ${i}`);
      yield `${name}: ${i}`;
      await new Promise((resolve) => setTimeout(resolve, dt));
    }
  } finally {
    console.log(`Closing ${name} (doing some cleanup)`);
    await new Promise((resolve) => setTimeout(resolve, 100));
  }
}

async function* caller() {
  // JS does a good job of propagating iterator close operation (i.e.
  // calling `.return()` an iterator is used in `yield*` or `for await`).
  yield* merge("iters-close-wait", generator("A", 222), generator("B", 555));
  // Available modes:
  // - "iters-noclose" (does not call inner iterators' `return` method)
  // - "iters-close-nowait" (calls `return`, but doesn't await nor throw)
  // - "iters-close-wait" (calls `return` and awaits for inners to finish)
}

(async () => {
  for await (const message of caller()) {
    if (message.includes("2")) {
      // This `break` closes the merged iterator, and the signal is
      // propagated to all inner iterators.
      break;
    }
    console.log(`Received from ${message}`);
  }
  console.log("Finishing");
})();

Result:

A yielded 0
B yielded 0
Received from A: 0
Received from B: 0
A yielded 1
Received from A: 1
A yielded 2
Closing A (doing some cleanup)
B yielded 1
Closing B (doing some cleanup)
Finishing

Inspired by

The alternative libraries mentioned below have one or more flaws. Mostly it's about inability to close the inner iterators once the merged iterator is closed, having a memory leak when one iterator finishes early, and about having an overcomplicated/slow code.

• https://github.com/reconbot/streaming-iterables/blob/master/lib/parallel-merge.ts
• https://github.com/fraxken/combine-async-iterators/blob/master/index.js
• https://github.com/vadzim/mergeiterator/blob/master/src/mergeiterator.ts
• https://github.com/hesher/mergen/blob/master/mergen.js
• https://github.com/ReactiveX/IxJS/blob/master/src/asynciterable/merge.ts
• https://github.com/laggingreflex/merge-async-iterators/blob/master/index.js

Situation: There are 6 different libraries to merge AsyncIterables with different bugs and corner cases.

Cueball: 6?! Ridiculous! We need to develop one universal library that covers everyone's use cases.

Ponytail: Yeah!

(Soon) Situation: There are 7 different libraries to merge AsyncIterables.