When to use the toAsync function?

Many functions in FxTS can handle both Iterable and AsyncIterable. For example, the find function can be used as follows.

const numbers = function* () {
  yield 1;
  yield 2;
  yield 3;
};

const asyncNumbers = async function* () {
  yield 1;
  yield 2;
  yield 3;
};

find((num) => num === 2, numbers()); // 2
find((num) => num === 2, asyncNumbers()); // Promise<2>

There is a special point to note. AsyncIterable can work fine whether the callback function is running synchronously/asynchronously, but You can not iterate over Iterable using asynchronous callback functions or manipulate types Iterable<Promise<T>>.

const promiseNumbers = function* () {
  yield Promise.resolve(1);
  yield Promise.resolve(2);
  yield Promise.resolve(3);
};

find((num) => Promise.resolve(num === 2), numbers()); // not work
find((num) => num === 2, promiseNumbers()); // not work

In order to handle async using FxTS, the value to iterate must be type of AsyncIterable. If the callback function is asynchronous or should handle Iterable<Promise<T>>, use the toAsync function to convert it to AsyncIterable.

await pipe(
  numbers(), // Iterable<number>
  toAsync, // AsyncIterable<number>
  find((num) => Promise.resolve(num === 2)),
);

await pipe(
  promiseNumbers(), // Iterable<Promise<number>>
  toAsync, // AsyncIterable<number>
  find((num) => Promise.resolve(num === 2)),
);