ok

Response: OK.

Indicates whether this response terminated with an "ok" status. If the response terminated with an HTTP status which was between 200 and 299, inclusive, then this returns true. For statuses 400 and above, this value is always false.

From MDN: "The ok read-only property of the Response interface contains a Boolean stating whether the response was successful (status in the range 200-299) or not."

See also: MDN, Response.ok.