The Serializer Component (Symfony Docs)
How to Use the Serializer (Symfony Docs)
Fractal - Output complex, flexible, AJAX/RESTful data structures
ScriptFUSION/Mapper: Transforms arrays using an object composition DSL.
API Transformer to Convert Your API Definition Files - APIMatic
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
JSONata