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How do `<iframe>` on a page communicate?

Topics
JAVASCRIPTHTML
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TL;DR

<iframe> elements on a page can communicate using the postMessage API. This allows for secure cross-origin communication between the parent page and the iframe. The postMessage method sends a message, and the message event listener receives it. Here's a simple example:

// In the parent page
const iframe = document.querySelector('iframe');
iframe.contentWindow.postMessage('Hello from parent', '*');
// In the iframe
window.addEventListener('message', (event) => {
console.log(event.data); // 'Hello from parent'
});

How do <iframe> on a page communicate?

Using the postMessage API

The postMessage API is the most common and secure way for iframes to communicate with each other or with their parent page. This method allows for cross-origin communication, which is essential for modern web applications.

Sending a message

To send a message from the parent page to the iframe, you can use the postMessage method. Here’s an example:

// In the parent page
const iframe = document.querySelector('iframe');
iframe.contentWindow.postMessage('Hello from parent', '*');

In this example, the parent page selects the iframe and sends a message to it. The second parameter, '*', is the target origin. It specifies the origin of the target window. Using '*' means the message can be received by any origin, but for security reasons, it's better to specify the exact origin.

Receiving a message

To receive a message in the iframe, you need to add an event listener for the message event:

// In the iframe
window.addEventListener('message', (event) => {
console.log(event.data); // 'Hello from parent'
});

The event object contains the data property, which holds the message sent by the parent page.

Security considerations

When using postMessage, it's crucial to consider security:

  • Specify the target origin: Instead of using '*', specify the exact origin to ensure that only messages from trusted sources are received.
  • Validate the message: Always validate the message content to prevent malicious data from being processed.

Example with target origin

Here’s an example with a specified target origin:

// In the parent page
const iframe = document.querySelector('iframe');
const targetOrigin = 'https://example.com';
iframe.contentWindow.postMessage('Hello from parent', targetOrigin);
// In the iframe
window.addEventListener('message', (event) => {
if (event.origin === 'https://parent.com') {
console.log(event.data); // 'Hello from parent'
}
});

In this example, the parent page sends a message only to https://example.com, and the iframe processes the message only if it comes from https://parent.com.

Further reading

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