Function batchedMap

batchedMap<T, U>(
    arr: T[],
    batch_size: number,
    fn: (batch: T[], index: number) => Promise<Nullable<U[]>>,
): Promise<U[]>
Asynchronously maps an array in batches and concatenates the results.
- If batch_size > 0, the array is split into consecutive batches of up to batch_size elements and processed sequentially.
- If batch_size === 0, the entire array is passed as a single batch (with index = 0).
The mapping function fn may return:
- an array of output items, which are appended to the final result, or
- null / undefined, which is simply treated as an empty array ([]).
Type Parameters
- T
  Type of input items.
- U
  Type of mapped output items.
Parameters
- arr: T[]
  The array to map.
- batch_size: number
  Items per batch. 0 means “use the entire array as one batch”. Must be a non-negative safe integer.
- fn: (batch: T[], index: number) => Promise<Nullable<U[]>>
  Async mapper invoked for each batch.
  
  batch: A slice of arr (or the entire array when batch_size === 0).
  
  index: The starting index of batch within arr.
Returns Promise<U[]>
A promise resolving to a flat array containing all mapped items.
Throws
If batch_size is negative or not a safe integer.
Example
```
// Normal batch mapping
const out = await batchedMap([1,2,3], 2, async(batch) => batch.map(x => x * 2));
// -> [2,4,6]
```
- Defined in function/batch.ts:79