How to use ETags to not refetch already retrieved S3 items

README.md

Refetching items from S3 can be quite memory and cpu intensive, and it can be unnecessary if the item has not been changed since. S3 allows for detecting changes to an item through the use of ETags. In the below code I am using the Etag of an item to see if it has changed since the last time it was fetched, if it was not changed since then the cached version of the item is returned instead.

const cache = new Map();
const eTags = new Map();
async function getObjectWithEtags({bucket, key} = {}) {
  const params = {
    Bucket: bucket,
    Key: key,
    IfNoneMatch: eTags.get(key),
  };
  try {
    const res = await exports
      .getClient()
      .getObject(params)
      .promise();
    eTags.set(key, res.ETag);
    cache.set(key, res);
    return res;
  } catch (e) {
    if (e.code == 'NotModified' || e.statusCode == '304') {
      return cache.get(key);
    }
    throw e;
  }
}