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• In a nutshell
• Guard
• 相关话题

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抓取 API provides an interface for fetching resources (including across the network). It will seem familiar to anyone who has used XMLHttpRequest , but it provides a more powerful and flexible feature set. This article explains some of the basic concepts of the Fetch API.

注意： This article will be added to over time. If you find a Fetch concept that you feel needs explaining better, let someone know on the MDN discussion forum ，或 MDN Web Docs room on 矩阵 .

In a nutshell

At the heart of Fetch are the Interface abstractions of HTTP Request s, 响应 s, 头 ，和 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.

服务工作者 is an example of an API that makes heavy use of Fetch.

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

Guard

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

当新 头 object is created using the Headers() 构造函数 , its guard is set to none (the default). When a Request or 响应 object is created, it has an associated 头 object whose guard is set as summarized below:

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

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

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

元数据

• 最后修改： Feb 18, 2020

相关话题

1. 抓取 API
2. 指南

   1. Using Fetch
   2. Fetch basic concepts
   3. Cross-global fetch usage
3. 接口

   1. Body
   2. 头
   3. Request
   4. 响应
4. 方法

   1. WindowOrWorkerGlobalScope.fetch()