Deep Analysis and Solutions for "Access is Denied" Error in jQuery AJAX CORS Requests on IE9

Problem Background and Phenomenon Description

In web development, Cross-Origin Resource Sharing (CORS) is a crucial mechanism for data communication between different origins. However, developers frequently encounter "Access is Denied" errors when using jQuery for AJAX requests in Internet Explorer 9, while the same code works correctly in other modern browsers. This issue particularly occurs with POST requests that don't require returned data, posing challenges for cross-browser compatibility.

Differences in IE9's CORS Implementation Mechanism

The root cause lies in significant differences between how IE9 and earlier versions implement CORS compared to other browsers. Most modern browsers use the standard XMLHttpRequest object to handle CORS requests, while IE8 and IE9 introduced the dedicated XDomainRequest object. This object has functional limitations, such as not supporting credentialed requests and having strict restrictions on request and response headers.

jQuery's AJAX method is primarily implemented based on XMLHttpRequest. When detecting cross-origin requests, it doesn't automatically switch to XDomainRequest, leading to compatibility issues in IE9. Even with jQuery.support.cors = true and crossDomain: true parameters set, jQuery's core library still cannot properly handle CORS requests in IE9.

Known jQuery Defect and Official Position

This issue is documented as a known defect in jQuery's bug tracking system. The jQuery development team has explicitly stated no plans to support XDomainRequest in the core library, considering it more suitable for plugin implementation. This design decision is based on multiple factors, including the functional limitations of XDomainRequest and IE10+'s support for standard CORS.

Solution: Extending AJAX Transport Mechanism via Plugins

The most effective solution is using specialized jQuery plugins to extend the AJAX transport mechanism, enabling proper handling of CORS requests in IE9. jQuery provides the $.ajaxTransport method, allowing developers to register custom transporter factory functions that are internally called by $.ajax to execute actual requests.

A mature solution is the jQuery-ajaxTransport-XDomainRequest plugin. This plugin works by detecting the browser environment and automatically using the XDomainRequest object instead of the standard XMLHttpRequest when making cross-origin requests in IE8 or IE9.

// Plugin usage example
// First include the jQuery-ajaxTransport-XDomainRequest.js file
// Then use $.ajax method as usual

$.ajax({
    url: "https://api.example.com/data",
    type: "POST",
    crossDomain: true,
    data: { key: "value" },
    success: function(response) {
        console.log("Request successful");
    },
    error: function(xhr, status, error) {
        console.error("Request failed:", error);
    }
});

The advantage of this plugin is its seamless integration into existing jQuery code. Developers don't need to modify their existing AJAX calls. The plugin automatically handles browser detection and transporter selection, ensuring code works correctly in both IE9 and other browsers.

Important Limitations of XDomainRequest

When using XDomainRequest, developers must be aware of several key limitations:

1. Protocol Consistency Requirement: Requests must use the same protocol as the hosting page. If the page uses HTTP, the target URL must also use HTTP; if the page uses HTTPS, the target URL must also use HTTPS. Mixed-protocol requests are rejected.
2. Functional Limitations: XDomainRequest doesn't support credentialed requests, cannot set custom request headers, and can only send text data.
3. Browser Compatibility: XDomainRequest only works in IE8 and IE9. IE7 and earlier don't support any form of CORS, while IE10 and later use standard XMLHttpRequest for CORS implementation.

Alternative Approach: JSONP Application Scenarios

For certain specific scenarios, JSONP (JSON with Padding) can serve as an alternative to CORS. JSONP achieves cross-origin requests by dynamically creating <script> tags, which aren't restricted by the same-origin policy. However, JSONP only supports GET requests and requires special server-side processing to wrap response data.

In the problem description, the user mentions considering JSONP as a last resort, even when no data needs to be returned. While this approach works, it's not elegant as it requires the server to return useless wrapped data, adding unnecessary overhead.

Practical Application Case

Referring to the second answer's practical application, developers successfully resolved CORS issues in IE9 using the aforementioned plugin. Below is an improved code example:

// After including the jQuery-ajaxTransport-XDomainRequest plugin
var fetchData = function() {
    $.ajax({
        cache: false,
        type: "GET",
        crossDomain: true,
        url: "https://api.example.com/endpoint",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function(data) {
            // Process response data
            processResponse(data);
        },
        error: function(xhr, status, error) {
            // Error handling
            handleError(error);
        }
    });
};

This code works correctly in Chrome, Firefox, IE10, and IE9, achieving true cross-browser compatibility.

Future Outlook and Best Practices

With IE10 and later versions providing full support for standard CORS, and the evolution of the modern browser market, IE9 compatibility issues are gradually diminishing. However, in enterprise environments requiring support for older IE versions, this issue remains relevant.

Best practices include:

1. Clearly defining the browser version range the project needs to support
2. Integrating specialized CORS compatibility plugins for projects requiring IE8/9 support
3. Adding appropriate browser detection and fallback handling in code
4. Considering feature detection rather than browser detection to determine available functionality
5. Encouraging users to upgrade to more modern browser versions when possible

By understanding the root causes of IE9 CORS issues and adopting appropriate solutions, developers can create web applications that operate stably across multiple browser environments, providing a consistent user experience.