javascript - 这个跨域ajax请求是如何工作的？

Question

我正在查看 this问题，其中有指向 http://hacks.mozilla.org/2011/03/the-shortest-image-uploader-ever/ 的链接其中有以下代码:

var fd = new FormData();
fd.append("image", file); // Append the file
fd.append("key", "6528448c258cff474ca9701c5bab6927");
// Get your own key: http://api.imgur.com/

// Create the XHR (Cross-Domain XHR FTW!!!)
var xhr = new XMLHttpRequest();
xhr.open("POST", "http://api.imgur.com/2/upload.json"); // Boooom!
xhr.onload = function() {
    // Big win!
    // The URL of the image is:
    JSON.parse(xhr.responseText).upload.links.imgur_page;
 }
 // Ok, I don't handle the errors. An exercice for the reader.
 // And now, we send the formdata
 xhr.send(fd);

这个跨域请求是如何工作的？我认为通常会有安全限制阻止人们这样做。

Answer 1

服务器响应 Access-Control-Allow-Origin 设置为允许跨域请求

Response Headers
Access-Control-Allow-Origin: *  
Cache-Control   max-age=604800
Connection  keep-alive
Content-Length  494
Content-Type    application/json

http://www.w3.org/TR/cors/#access-control-allow-origin-response-hea

http://hacks.mozilla.org/2009/07/cross-site-xmlhttprequest-with-cors