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Fetch is a modern concept equivalent to XMLHttpRequest. It offers a lot of the same functionality as the XMLHttpRequest, but is designed to be more extensible and efficient. This article explains some of the basic concepts of the Fetch API.
In a nutshell
At the heart of Fetch are the Interface abstractions of HTTP
Body payloads, along with a
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
Guard is a feature of
Headers objects, with possible values of
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
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
A header's guard affects the
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