A full REST API like with Java's jax-rs contains definitions for defining a path for a resource, uses the full GET, POST, PUT requests. But, typically when I encounter a REST API, it is typically a standard HTTP GET request and the response is a JSON output.

The browser-native fetch API responses include an ok boolean that indicates whether any non-success status was returned. You can always check for anything outside the 2XX range using comparison operators.

One of the most popular ways to send and receive data is through RESTful APIs, using JSON data format. This article will provide an in-depth understanding of how JSON works with RESTful APIs, plus tips and tricks to help you avoid common pitfalls.

Parsing JSON API responses is a fundamental skill for developers working with APIs. It involves converting the JSON-formatted string received from an API into a data structure that can be manipulated and used within an application.

That would be a good method to have, response.isJson(). You could check for the content-type of the response, as shown in this MDN example: const contentType = response.headers.get("content-type"); if (contentType && contentType.indexOf("application/json") !== -1) { return response.json().then(data => { // The response was a JSON object.