Add custom serializer option

Currently JSONB.stringify/JSONB.parse is used for serializing/deserializing to support a wide range JavaScript Types.

Allowing users to set a custom a custom serializer function will allow for cleaner usage with custom data structures and enable performance improvements for people who don’t need all the type support JSONB provides.

Syntax should look something like:

const Keyv = require('keyv');
const { serialize, deserialize } = require('my-serializer');

const keyv = new Keyv({ serialize, deserialize });

Previous discussion in #22

[lukechilds]

Or to a rabbit on the internet.

[lukechilds]

Closed by https://github.com/lukechilds/keyv/pull/33