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

How to Use the Serializer (Symfony Docs)

The Serializer Component (Symfony Docs)

API Transformer to Convert Your API Definition Files - APIMatic

ScriptFUSION/Mapper: Transforms arrays using an object composition DSL.

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