跨源資源共享（CORS）是一種允許來自web瀏覽器中另一個域的受限資源的機制。

假設，如果您在HTML5演示部分中單擊HTML5-video player。它會得到攝像機的許可。如果用戶允許該權限，則只有它會打開相機，否則它不會爲web應用程式打開相機。

Making a CORS request

這裡Chrome、Firefox、Opera和Safari都使用XMLHttprequest2對象，而Internet Explorer使用類似的XDomainRequest對象object。

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');
}

Event handles in CORS

Example of onload or onerror event

xhr.onload = function() {
   var responseText = xhr.responseText;
   
   // process the response.
   console.log(responseText);
};

xhr.onerror = function() {
   console.log('There was an error!');
};

Example of CORS with handler

下面的示例將顯示makeCorsRequest（）和onload處理程序的示例