It is calculated asynchronously. Response body is lazy and calling .json() on it consumes it and returns promise. That is API of WHATWG fetch and many other JS

It is calculated asynchronously. Response body is lazy and calling .json() on it consumes it and returns promise. That is API of WHATWG fetch and many other JS HTTP clients https://fetch.spec.whatwg.org/#body-mixin https://fetch.spec.whatwg.org/#body-mixin