HTML5 and CSS3, Second Edition

Tip 31

Tip 31. Talking across Domains

Client-side web applications have traditionally been restricted from talking

directly to scripts on other domains—a restriction designed to protect users.

This is known as the same-origin policy,^3 and while it protects users, it makes

our work more difficult if we have legitimate reasons to make two separate

sites communicate. There are numerous clever ways around this restriction,

including the use of server-side proxies and URL hacks. But now there’s a

better way.

Cross-Document Messaging, or Web Messaging,^4 is an API that makes it pos-

sible for scripts hosted on different domains to pass messages back and forth.

For example, we can have a form on http://support.awesomecompany.com post content

to another window or <iframe> whose content is hosted on http://www.awesomecom-

pany.com. For our current project, we need to do just that.

AwesomeCo’s new support site will have a contact form, and the support

manager wants to list all the support contacts and their email addresses next

to the contact form, like in the following figure.

Figure 33—Our completed support site

3. https://developer.mozilla.org/en/Same_origin_policy_for_JavaScript.


4. http://www.w3.org/TR/webmessaging/#web-messaging

report erratum • discuss

Talking across Domains • 213

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