55Implement batch API calls with maximum batch size and interval

A product list renders 60 cards and each card asks for its price: getPrice(id). The API has a batch endpoint, POST /prices with up to 25 ids. Turning 60 calls into 3 requests needs a batcher.

Implement createBatcher({ sendBatch, maxBatchSize = 25, maxWaitMs = 10 }). It returns:

• load(item): returns a promise for that item's result.
• flush(): send the current batch now.

sendBatch(items) receives an array of items and must resolve to an array of results in the same order. A batch is sent when it reaches maxBatchSize items or maxWaitMs after its first item was added, whichever comes first.

const prices = createBatcher({
  maxBatchSize: 25,
  sendBatch: (ids) => fetch("/prices", { method: "POST", body: JSON.stringify(ids) }).then((r) => r.json()),
});
const price = await prices.load(productId);

Requirements

• Each load promise resolves with its own result from the batch.
• If sendBatch rejects, every promise in that batch rejects with the same error. Other batches are unaffected.
• If sendBatch returns a different number of results than items, reject that batch's promises with an error.
• Several batches may be in flight at once.