How to understand Js cross-domain

What is cross-domain

The browser has the same origin policy and does not allow ajax to access the interface of other domains

Cross-domain conditions: protocol, domain name, port, if there is one difference, it is considered cross-domain

http The default port is 80

https The default port is 443

There are three tags that allow cross-domain loading of resources

// Can be used for statistics. The statistics website may be from other domains

// Can be used for JSONP, as well as Use CDN

// You can use CDN, CDN is also used in other domains

Several ways to cross domains

1. JSONP cross-domain data request

JSONP implementation principle

1. Load http://www.baidu.com/test.html

2. Not necessarily served There is really a test.html file on the end

　3. The server can dynamically generate a test.html file according to the request and return

　4. The same principle applies to

##

<script>
　　window.callback = function (data) {
　　　　console.log(data);　　// 这是跨域得到的信息
　　}
</script>
<script src="http://www.baidu.com/api.js"></script>
<!-- api.js 内容是： callback({x: 100, y: 200}) -->

2. Set http header on the server. This is a way to solve cross-domain problems in the future. Trend

// 注意：不同后端语言的写法可能不一样

// 第二个参数填写允许跨域的域名，* 代表允许所有域，不建议直接写 *
response.setHeader("Access-Control-Allow-Origin", "http://a.com, http://b.com");

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