The Serializer Component (Symfony Docs)

How to Use the Serializer (Symfony Docs)

Fractal - Output complex, flexible, AJAX/RESTful data structures

Jsonnet - The Data Templating Language

neilflatley/jsonpath-mapper: A json to json transformation utility with a few nice features to use when translating for example API responses into a domain object for use in your domain-driven JavaScript applications