flatten.ts

function *flatten<T>(iterable: Iterable<T>, depth: number = Infinity): Iterable<T> {
  if (depth === 0) { return iterable; }
  
  for (let item of iterable) {
    if (typeof item[Symbol.iterator] === 'function') {
      yield *flatten(item, depth - 1);
    } else {
      yield item;
    }
  }
}
  
const arr = [1,[2],[3,4]];
const result = flatten(arr);
  
for (let item of result) {
  console.log(result);  
}

flattenasync.ts

function *flattenAsync<T>(iterable: AsyncIterable<T>, depth: number = Infinity): AsyncIterable<T> {
  if (depth === 0) { return iterable; }
  
  for await (let item of iterable) {
    if (typeof item[Symbol.asyncIterator] === 'function') {
      yield *flatten(item, depth - 1);
    } else {
      yield item;
    }
  }
}