Fetch API: Basic concepts

In a nutshell

At the heart of Fetch are the Interface abstractions of HTTP Requests, Responses, Headers, and Body payloads, along with a global fetch method for initiating asynchronous resource requests. Because the main components of HTTP are abstracted as JavaScript objects, it is easy for other APIs to make use of such functionality.

Service Workers is an example of an API that makes heavy use of Fetch.

Fetch takes the asynchronous nature of such requests to one step further. The API is completely Promise-based.

Guard

Guard is a feature of Headers objects, with possible values of immutable, request, request-no-cors, response, or none, depending on where the header is used.

When a new Headers object is created using the Headers() constructor, its guard is set to none (the default). When a Request or Response object is created, it has an associated Headers object whose guard is set as summarized below:

new object's type	creating constructor	guard setting of associated Headers object
Request	Request()	request
	Request() with mode of no-cors	request-no-cors
Response	Response()	response
	error() or redirect() methods	immutable

A header's guard affects the set(), delete(), and append() methods which change the header's contents. A TypeError is thrown if you try to modify a Headers object whose guard is immutable. However, the operation will work if

• guard is request and the header name isn't a forbidden header name .
• guard is request-no-cors and the header name/value is a simple header .
• guard is response and the header name isn't a forbidden response header name .