How to Use window.postMessage() to Send Messages Between Apps in the Same Browser Window

Learn how to effectively send messages between a parent Angular app and a child React app using `window.postMessage()`. Get practical examples and troubleshooting tips.
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This video is based on the question https://stackoverflow.com/q/71885449/ asked by the user 'Ethan Smith' ( https://stackoverflow.com/u/10235624/ ) and on the answer https://stackoverflow.com/a/71885641/ provided by the user 'Cesare Polonara' ( https://stackoverflow.com/u/13478993/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Effectively Use window.postMessage() to Send Messages Between Apps in the Same Browser Window

In modern web development, creating integrated applications that communicate seamlessly is essential. If you’re working with multiple web applications loaded within the same browser instance, sharing data between them can sometimes be tricky. A common scenario arises when you have a parent Angular app that contains a child React app within an iframe and you want these applications to exchange messages.

If you’ve tried using window.postMessage() but found that the messages don’t seem to be sent or received correctly, don’t worry! In this post, we’ll break down the process to ensure your apps communicate effectively.

Understanding window.postMessage()

The window.postMessage() method provides a controlled mechanism to communicate between Window objects, such as between a parent document and iframes. This communication happens across different origins, and it's essential for situations where an iframe is loaded within another web application.

Problem Scenario

In our case, we want messages to be exchanged between:

Parent App: Built with Angular

Child App: Built with React, and loaded inside the parent app as an iframe.

You may have tried the following code in your applications:

Parent App Code:

[[See Video to Reveal this Text or Code Snippet]]

Child App Code:

[[See Video to Reveal this Text or Code Snippet]]

What Went Wrong?

Many developers encounter issues with the communication not going through as expected. When checking the console, the error messages can be misleading or unclear. So what can you do differently?

The Solution: Properly Dispatching Messages

The key to successful communication between the parent and child apps is ensuring you’re sending messages from the correct window object. Instead of using window.postMessage() directly from the parent application’s window, you should be dispatching messages from the child’s context.

Here’s How to Do It Correctly

Access the iframe’s content window in the parent app:
Instead of window.postMessage, select the iframe and use its content window.

Use the following code in your Parent App:

[[See Video to Reveal this Text or Code Snippet]]

Updated Child App Listener

Keep your child app code intact to listen for incoming messages:

[[See Video to Reveal this Text or Code Snippet]]

Best Practices for Using window.postMessage()

Origin Security: Instead of using "*" as the target origin, specify the exact domain from which you expect to receive messages for enhanced security.

Validation: Always validate messages in the child app by checking event.origin before processing the message to ensure it comes from a trusted source.

Error Handling: Implement comprehensive error handling in both apps to catch and log any issues that arise during the communication.

Conclusion

By following the corrected approach using contentWindow of the iframe, your parent Angular app and child React app should communicate effectively using window.postMessage(). This method is a powerful tool for managing interactions between separate applications, providing a smooth user experience.

Implement these changes today, and you’ll find that cross-app messaging isn't just possible but straightforward!

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