The purpose of typed_params is to declare and enforce type constraints on your Rails API endpoints.

NOTE: We use 'parameter' and 'attribute' somewhat interchangably, as this is the same convention used by Rails (notice the usage of the two words throughout that documentation).

Imagine you have a Foo model/resource, and a FoosController. You want to enforce a certain params structure for requests to foos#create.

class FoosController < ActionController::API
  # enforce json-api format
  CreateFooParams = {
    data: {
      attributes: {
        name:  String,
        email: String
      }
    }
  }

  assert_params_format CreateFooParams, on: :create

  def create
    # If you reach this line, then the request body adheres to the type
    # declarations of CreateFooParams. Proceed with confidence!
  end
end

The gem is available as open source under the terms of the MIT License.