Shared data

If you have state that should be shared between all pages. Just use common Rails patterns. Simply put them in your layout.

For example, in the generated application.json.props

path = request.format.json? ? param_to_dig_path(params[:props_at]) : nil

json.data(dig: path) do
+  json.header do
+    json.email "foo@foo.com"
+  end

  yield json
end

In the above, every page that gets rendered will have header as part of the [page response].

You can also use partials:

app/
|-- controllers/
|-- views/
|   |-- shared/
|   |   |-- _header.json.props

path = request.format.json? ? param_to_dig_path(params[:props_at]) : nil

json.data(dig: path) do
+  json.header(partial: 'shared/header') do
+  end
  yield json
end

Advanced functionality

The above results in duplicate JSON nodes across our pages:

{
  pages: {
    "/posts": {
      data: {
        header: {
          email: "foo@foo.com"
        }
      }
    },
    "/comments": {
      data: {
        header: {
          email: "foo@foo.com"
        }
      }
    },
  }
}

For most cases where you don't need client side updates, this may be good enough. Its a fine tradeoff for simplicity, but if you need referential identity -- you want to update that header and have that reflected across pages, check out Fragments.