Entity reference

Schema:

{
  "type": "object",
  "properties": {
    "type": {"type": "string", "pattern": "^form-field$"},
    "id": {"type": "string", "pattern": "^[0-9a-f]{8}-([0-9a-f]{4}-){3}[0-9a-f]{12}$"},
    "machine_name": {"type": "string"},
    "data": {
      "type": "object",
      "properties": {
        "uid": {"type": "string","pattern": "^form-entity-reference$"},
        "title": {"type": "string"},
        "value": {
          "type": "object",
          "properties": {
            "entity_type": {"type": "string"},
            "view_mode": {"type": "string"},
            "entity": {"type": "string"}
          }
        }
      }
    }
  },
  "required": ["id", "type", "data"]
}

Example output:

Given "Entity reference" form element on a component set up like so

And used on Node layout canvas like so

JSON:API output for this form field will be this:

{
  "type": "form-field",
  "id": "954f324a-d706-4f72-9cfa-e74d02aa692f",
  "machine_name": "entity-reference",
  "data": {
    "uid": "form-entity-reference",
    "title": "Entity reference",
    "value": {
      "entity_type": "node",
      "view_mode": "full",
      "entity": "6549c652-a789-410a-aeb5-534b8458a45b",
      "jsonapi_link": "https://sitestudiodocs.acquia.com/jsonapi/node/layout_page/6549c652-a789-410a-aeb5-534b8458a45b"    }
  }
}

In this example "value" property contains "jsonapi_link" property as the "Entity reference" form field stores a reference to a Node. The value of "jsonapi_link" property is an absolute url pointing to a JSON:API Resource for referenced node "Example node".