Proxying Requests

Often third-party scripts are added to the page by appending a script tag, such as:

var script = document.createElement('script');
script.url = 'http://some-third-party-script.com/tracking.js';
document.head.appendChild(script);

When the <script> element is appended to the <head> using this traditional approach, the script’s HTTP response does not require Cross-Origin Resource Sharing (CORS) headers.

However, because Partytown requests the scripts within a web worker using fetch(), then the script’s response requires the correct CORS headers.

Many third-party scripts already provide the correct CORS headers, but not all do. For services that do not add the correct headers, then a reverse proxy to another domain must be used in order to provide the CORS headers.

CORS Response Header

Access-Control-Allow-Origin: *

Configuring URL Proxies

When executed from within Partytown, every URL is resolved by Partytown, which also gives users the ability to inspect and modify any and every URL being resolved. The resolveUrl() config can be used to check for a specific URL, and optionally return the proxied URL instead. For example, in the code below we’re checking if the URL to be resolved is for Google Analytics, and if so, return a different URL that points to a reverse proxy.

Vanilla Config Example

partytown = {
  resolveUrl: function (url, location, type) {
    if (type === 'script') {
      var proxyUrl = new URL('https://my-reverse-proxy.com/');
      proxyUrl.searchParams.append('url', url.href);
      return proxyUrl;
    }
    return url;
  },
};

Please see the integration guides and configuration for more information.

Reverse Proxy

Below are a few examples of reverse proxies that could be used.

Builder.io also provides a hosted reverse proxy API you can use for Partytown with a free account

partytown = {
  resolveUrl: function (url, location, type) {
    if (type === 'script') {
      const proxyUrl = new URL('https://cdn.builder.codes/api/v1/js-proxy');
      proxyUrl.searchParams.append('url', url.href);
      proxyUrl.searchParams.append('apiKey', YOUR_BUILDER_PUBLIC_API_KEY);  
      return proxyUrl;
    }
    return url;
  },
};

Serving Resources Locally

Another option to work around the Access-Control-Allow-Origin issue without using a proxy is to serve your third-party JavaScript and other resources locally.

So instead of inserting something like this on your page for Google Analytics:

<!-- Google Analytics -->
<script type="text/partytown">
  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
  })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

  ga('create', 'UA-XXXXX-Y', 'auto');
  ga('send', 'pageview');
</script>
<!-- End Google Analytics -->

You instead download the analytics.js JavaScript resource, and serve it locally from your domain like this:

<!-- Google Analytics -->
<script type="text/partytown">
  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
  })(window,document,'script','https://example.com/analytics.js','ga');

  ga('create', 'UA-XXXXX-Y', 'auto');
  ga('send', 'pageview');
</script>
<!-- End Google Analytics -->

Since you are serving the resource from your own server, you control the headers that come along with it, including the Access-Control-Allow-Origin header that the proxy work-around is sometimes needed for.

As a bonus, serving these resources locally is one less DNS lookup that needs to happen before the client can load your site.

You can manually download these self-hosted third-party JavaScript resources, or you can use a webpack plugin like save-remote-file-webpack-plugin to do it as part of your build process:

const SaveRemoteFilePlugin = require('save-remote-file-webpack-plugin');
module.exports = {
  plugins: [
    new SaveRemoteFilePlugin([
      {
        url: 'https://google-analytics.com/analytics.js',
        filepath: 'js/analytics.js',
      },
    ]),
  ],
};