@cocalc/local-storage-lru

LRU Cache for Browser's Local Storage

   

Problem

Saving an increasing number of key/value pairs in localStorage causes it to fill up at some point. From that point on, adding/modifying throws an exception.

Solution

Keep track of last n recently accessed/modified keys. If an exception occurs, randomly remove a few keys which aren't in that list and also – optionally – only whitelisted ones. Then try again storing the new value.

Benefits

• The entire overhead is one additional key/value pair storing the pointers to these LRU keys.
• The keys and values you try to store are not modified.

Design Goals

• robust: no exceptions are thrown (only if there is a problematic key)
• universal: also supports storing objects, Date, BigInt, arrays etc..
• backwards compatible: if you already store string values, they're not modified. You can even tell it to attempt parsing existing JSON values.

Usage

This is how to instantiate the wrapper class.

Options:

• recentKey (optional): the string of the key, under which the list of recently accessed keys is stored.
• maxSize (optional): the maximum number of keys to keep in the list of recently used keys. A larger list reduces the chances of deleting an "important" key, but at the same time, overall more storage is used.
• isCandidate (optional): a function that takes a key and returns true if the key is a candidate for deletion. By default, any key except for the recentKey is a candidate. Optional second argument is the array of all recent keys.
• fallback (optional, default false): if true, localStorage is checked if it works. If not, data is stored in a mockup storage with limited space.
• serializer (optional): by default JSON.stringify, but you can use your own.
• deserializer (optional): counterpart to the above, by default JSON.parse.
• parseExistingJSON (optional): if true, it tries to deserialize already existing JSON values.
• typePrefixes (optional): prefixes to serialized values if they're not stored a strings – somehow, we have to mark values if they are a complex object...
• typePrefixDelimiter (optional): string appended to each typePrefix to separate from the serialized value – default: \0

// simple:
const storage = new LocalStorageLRU();

// with options:
function candidate(key: string): boolean {
  if (key.startsWith('preserved-')) {
    return false;
  }
  return true;
}

const storage = new LocalStorageLRU({
  recentKey: RECENTLY_KEY,
  maxSize: RECENTLY_KEEP,
  isCandidate: candidate,
  fallback: true,
});

// set/get/delete
storage.set('foo', 'bar');
storage.get('foo') == 'bar'; // true
storage.delete('foo');

// iterate
storage.set('key1', '1');
storage.set('key2', '2');
storage.set('key3', '3');
const entries: [string, any][] = [];
for (const [k, v] of storage) {
  entries.push([k, v]);
}
entries; // equals: [[ 'key1', '1' ], [ 'key2', '2' ], [ 'key3', '3' ]]

For more, check out the documentation or the tests.

Development

The setup follows this step-by-step guide.

Release

There are some hooks registered, see link above, to check for clean git tree, all tests passing and linting.

npm version patch
npm publish

License

Apache 2.0