HTML - 跨域资源共享 (CORS)

跨域资源共享 (CORS) 是一种机制，允许在 web 浏览器中加载来自另一个域的受限资源。

例如，假设我们点击 HTML5 演示部分中的 HTML5 视频播放器。首先，它会请求摄像头权限，如果用户允许该权限，则只会打开摄像头，否则不会。

发起 CORS 请求

现代浏览器如 Chrome、Firefox、Opera 和 Safari 都使用 XMLHttprequest2 对象，而 Internet Explorer 使用类似的 XDomainRequest 对象。

function createCORSRequest(method, url) {
   var xhr = new XMLHttpRequest();
   
   if ("withCredentials" in xhr) {
      // Check if the XMLHttpRequest object has a "withCredentials" property.
      // "withCredentials" only exists on XMLHTTPRequest2 objects.
      xhr.open(method, url, true);
   } else if (typeof XDomainRequest != "undefined") {
      // Otherwise, check if XDomainRequest.
      // XDomainRequest only exists in IE, and is IE's way of making CORS requests.
      xhr = new XDomainRequest();
      xhr.open(method, url);
   } 
   else {
      // Otherwise, CORS is not supported by the browser.
      xhr = null;
   }
   return xhr;
}
var xhr = createCORSRequest('GET', url);
if (!xhr) {
   throw new Error('CORS not supported');
}

CORS 中的事件处理程序

onload 或 onerror 事件示例

xhr.onload = function() {
   var responseText = xhr.responseText;
   // process the response.
   console.log(responseText);
};
xhr.onerror = function() {
   console.log('There was an error!');
};

带处理程序的 CORS 示例

下面的示例将展示 makeCorsRequest() 和 onload 处理程序的示例

// Create the XHR object.
function createCORSRequest(method, url) {
   var xhr = new XMLHttpRequest();
   if ("withCredentials" in xhr) {
      
      // XHR for Chrome/Firefox/Opera/Safari.
      xhr.open(method, url, true);
   } else if (typeof XDomainRequest != "undefined") {
      
      // XDomainRequest for IE.
      xhr = new XDomainRequest();
      xhr.open(method, url);
   } else {
      
      // CORS not supported.
      xhr = null;
   }
   return xhr;
}
// Helper method to parse the title tag from the response.
function getTitle(text) {
   return text.match('<title>(.*)?</title>')[1];
}

// Make the actual CORS request.
function makeCorsRequest() {
   // All HTML5 Rocks properties support CORS.
   var url = 'https://tutorialspoint.com';
   var xhr = createCORSRequest('GET', url);
   if (!xhr) {
      alert('CORS not supported');
      return;
   }
   
   // Response handlers.
   xhr.onload = function() {
      var text = xhr.responseText;
      var title = getTitle(text);
      alert('Response from CORS request to ' + url + ': ' + title);
   };
   xhr.onerror = function() {
      alert('Woops, there was an error making the request.');
   };
   xhr.send();
}