Combine one or more async generators in TypeScript

mergeGenerators.ts

/**
 * Merge multiple generators into a single generator.
 */
function mergeGenerators<T>(generators: AsyncGenerator<T>[]): AsyncGenerator<T>

/**
 * Merge multiple generators into a single generator. Whenever a generator
 * yields, the `{done, value}` object is passed to `mapResult` to transform the
 * value that will be yielded.
 */
function mergeGenerators<T, U = T>(
  generators: AsyncGenerator<T>[],
  mapResult: (result: IteratorResult<T>) => U,
): AsyncGenerator<NonNullable<U>>

async function* mergeGenerators<T, U>(
  generators: AsyncGenerator<T>[],
  mapResult?: (result: IteratorResult<T>, index: number) => U,
) {
  type IteratorPromise = Promise<{
    result: IteratorResult<T>
    index: number
  }>

  const promises: (IteratorPromise | null)[] = generators.map((gen, index) =>
    gen.next().then(result => ({ result, index })),
  )

  while (promises.some(Boolean)) {
    const { result, index } = (await Promise.race(promises.filter(Boolean)))!

    if (mapResult) {
      const value = mapResult(result, index)
      if (value != null) {
        yield value
      }
    } else if (!result.done) {
      yield result.value
    }

    promises[index] = result.done
      ? null
      : generators[index].next().then(result => ({ result, index }))
  }
}