Overcoming CORS Challenges in Optimizely Experiments: A Simple Solution

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My daily routine involves conducting numerous Optimizely experiments for testing new features, interfaces, and ideas, ultimately enhancing user experience across the board. However, a recurring challenge in this process is navigating the complexities of Cross-Origin Resource Sharing (CORS) policies, especially when deploying detached components on platforms like Heroku. The CORS policy can become a significant hurdle when integrating components hosted on different domains, a common scenario for web developers striving to enhance their applications with external resources.

The CORS issue

CORS policies are designed to safeguard the web by preventing malicious websites from accessing resources and data from another domain without permission. While these policies are indispensable for maintaining web security, they can also impede the seamless integration of beneficial third-party services and APIs—particularly when attempting to embed content from one domain (like a Heroku-deployed app) into a page hosted on another domain.

A Simple Solution with postMessage

To bridge this gap in my Optimizely experiments, I’ve turned to the postMessage API, a powerful tool in the web developer’s arsenal for secure cross-origin communication. This method allows for data exchange between an iframe and its parent document, circumventing CORS restrictions without compromising security.

Step-by-Step Integration
  1. Identifying the Need: Recognizing when CORS policies block the integration of cross-origin resources is the first step. This often manifests as errors or blocked requests when attempting to embed or interact with components from different domains.
  2. Leveraging postMessage: The postMessage API facilitates messaging across origins. By implementing this, I can securely communicate between an iframe (hosting the Heroku-deployed component) and its parent page on another domain.

In the iframe:

// Sending a message to the parent window
parent.postMessage('Hello from the iframe!', 'https://parent-domain.com');

In the Parent Document (via Optimizely):

// Listening for messages from the iframe
window.addEventListener('message', (event) => {
    if (event.origin !== 'https://iframe-domain.com') {
        // Ensure the message is from a trusted source

    console.log('Message received from iframe:', event.data);

Optimizely Experiment Integration: Incorporating the postMessage technique into Optimizely experiments allows for seamless testing of new features that require cross-domain communication and tracking events. This approach not only resolves CORS issues but also ensures that experiments can run without hitches, offering a richer, more interactive user experience.

Security First: A critical aspect of using postMessage is ensuring the security of the communication. Always verify the origin of incoming messages and handle the data with care, to safeguard against potential vulnerabilities.

Comprehensive Testing: Before rolling out experiments broadly, conducting thorough tests across various browsers and devices is essential. This ensures that the cross-origin communication works flawlessly, providing valuable insights without technical interruptions.


This solution reinforces the idea that simplicity, when applied with insight, can unlock new possibilities and streamline the path to achieving our digital experimentation goals. In the intricate web of development challenges, the simplest solutions are sometimes the most powerful and transformative.