The Response.json() static method permits constructing a Response object with the body populated with a JavaScript object serialized as JSON.

Links:

Explainer: https://docs.google.com/document/d/1dTycWmyxLZNGTBW93fvtf1IQahx-vNwgu94yT1x9K50/edit
Spec: https://fetch.spec.whatwg.org/#ref-for-dom-response-json
ChromeStatus entry: https://chromestatus.com/feature/5197912798658560