Cross-origin requests failing when loading JavaScript files from external domains

Other Questions JavaScript
1

I’m working on a browser plugin that needs to read JavaScript file contents from different domains. Same-origin requests work fine with regular XMLHttpRequest, but cross-domain requests keep failing even when I try to implement CORS properly.

// Helper function to set up cross-origin request
function setupCrossOriginRequest(httpMethod, targetUrl) {
  var request = new XMLHttpRequest();
  if ("withCredentials" in request) {
    // Modern browsers support
    request.open(httpMethod, targetUrl, true);
  } else if (typeof XDomainRequest != "undefined") {
    // Internet Explorer fallback
    request = new XDomainRequest();
    request.open(httpMethod, targetUrl);
  } else {
    // No CORS support available
    request = null;
  }
  return request;
}

// Execute the cross-domain request
function executeRemoteRequest(fileUrl) {
  var request = setupCrossOriginRequest('GET', fileUrl);
  if (!request) {
    console.log('Cross-origin requests not supported');
    return;
  }

  request.onload = function() {
    return request.responseText;
  };

  request.onerror = function() {
    console.log('Request failed with error');
  };

  request.send();
}

When I call executeRemoteRequest('https://example.com/script.js'), the browser throws an error about missing Access-Control-Allow-Origin header and blocks the request. How can I properly fetch JavaScript files from external domains?

2

Your executeRemoteRequest function doesn’t handle async operations properly. You can’t return from the onload callback - it just won’t work. I hit this exact issue building a content aggregator extension last year. Switch to fetch() with proper promises and make sure your extension has the right host permissions. Quick tip: CDNs like jsDelivr and unpkg have permissive CORS policies if you’re grabbing popular libraries. For your own domains, set Access-Control-Allow-Origin headers on the server. Third-party domains? You’ll need a proxy or use extension privileges. Drop the XDomainRequest fallback - IE’s dead and you probably don’t need legacy support.

3

You can’t bypass CORS from client-side JavaScript when the target server doesn’t allow cross-origin requests. Your code’s fine, but it’ll fail if the external domain hasn’t set Access-Control-Allow-Origin headers. Since this is for a browser plugin, you’ve got options regular web pages don’t. Extensions can request special permissions in their manifest to bypass CORS. For Chrome extensions, declare the domains in the permissions array and use the background script context. Or set up a proxy server on your domain that fetches the external JS files and serves them with proper CORS headers. Adds an extra network hop but gives you full control over response headers. The proxy works reliably across all browsers without extension-specific code.

4

check ur manifest permissions - browser plugins run in diff security contexts than reg webpages. ur onload function won’t return anything useful since it’s async. u need a callback param to handle the response.