getter = (obj, hashFunction = DEFAULTS.HASH_FUNCTION) => {
	let innerMap;
	if (typeof hashFunction === "string") {
		innerMap = mapStrings.get(hashFunction);
		if (innerMap === undefined) {
			const newHash = new LazyHashedEtag(obj, hashFunction);
			innerMap = new WeakMap();
			innerMap.set(obj, newHash);
			mapStrings.set(hashFunction, innerMap);
			return newHash;
		}
	} else {
		innerMap = mapObjects.get(hashFunction);
		if (innerMap === undefined) {
			const newHash = new LazyHashedEtag(obj, hashFunction);
			innerMap = new WeakMap();
			innerMap.set(obj, newHash);
			mapObjects.set(hashFunction, innerMap);
			return newHash;
		}
	}
	const hash = innerMap.get(obj);
	if (hash !== undefined) return hash;
	const newHash = new LazyHashedEtag(obj, hashFunction);
	innerMap.set(obj, newHash);
	return newHash;
}
@param {HashableObject} obj object with updateHash method
@param {(string | HashConstructor)=} hashFunction the hash function to use
@returns {LazyHashedEtag} etag