JSON.parse and type the return value as the type you're expecting:
data can be any type you assume the JSON is. As you can see, this compiles to a straightforward
JSON.parse call. As with regular JS, this is convenient, but has no guarantee that e.g. the data is correctly shaped, or even syntactically valid. Slightly dangerous.
The Js.Json module provides slightly safer, low-level building blocks for power users who want to parse JSON on a per-field basis. See the examples in the API docs.