body

Request: Body.

Specifies, if any, a ReadableStream which holds the body of a FetchRequest. Body data may only be provided when the method for the request allows a body, such as POST, PUT, and so forth.

From MDN: "The read-only body property of the Request interface contains a ReadableStream with the body contents that have been added to the request. Note that a request using the GET or HEAD method cannot have a body and null is returned in these cases."

See also: MDN, Request.body.