Great for testing/debugging: IE9’s Document Modes and JavaScript (via the IEBlog)

Internet Explorer 9 standards document mode enables the same markup and same
script to work across browsers. You should use Internet Explorer 9 standards
document mode to take advantage of new features in the
ECMAScript, Fifth Edition standard (ES5) and IE9’s
enhanced DOM programmability. However, to maintain fidelity with
older versions of IE, the JavaScript functionality supported in IE9’s Quirks
mode, IE7 standards mode, and IE8 standards mode differs somewhat from IE9
standards mode. You can use these compatibility modes and the F12 developer
tools to migrate your site to use the new ES5 features on your own schedule,
while your site continues to run in IE9.

In two blog posts – Testing
sites with Browser Mode vs. Doc Mode and
IE’s Compatibility Features for Site Developers – Marc Silbey
explains how to take advantage of document modes in Internet Explorer 9. He
also discusses how to use Browser Mode and Document Mode to test sites for
IE9 and previous IE versions. In this post, we’ll explore what developers
need to know about how the document modes in IE9 affect JavaScript code.

Document Modes and JavaScript Support

As mentioned
in a previous post, you (the developer) control the document mode
that IE will use when rendering your site by using the DOCTYPE and
X-UA-Compatible Meta tag or HTTP Header. Chakra, the new JavaScript engine in
IE9, uses the document mode to determine the JavaScript features to support.
The table below summarizes Chakra’s JavaScript support under the four IE9
document modes. For information on how to set the document mode, see our post
on
IE’s Compatibility Features for Site Developers and the
Determining Document Mode diagram for IE9.

Document Mode	Description
IE9 standards	IE9 standards document mode is the default if a Web page uses a standards-compliant DOCTYPE and doesn’t specify an X-UA-Compatible meta tag. In this mode, IE supports ECMAScript, Fifth Edition features, enhanced DOM programmability, and removes some of the key differences between our IE8 JavaScript implementation and the ECMAScript, Third Edition Specification.
IE8 standards	IE8 standards document mode supports the JavaScript additions we made in IE8 to implement portions of the then-draft ES5 standard, such as native JSON support and accessor support on DOM objects. This mode also supports the changes made in IE8 to fix some key issues raised by developers.
IE7 standards	IE7 standards document mode supports the JavaScript functionality that was available in IE7, including the Microsoft extensions supported in the IE7 standards mode in IE7 and IE8.
Quirks	Quirks mode supports the JavaScript functionality of IE6, and the Quirks mode of IE7 and IE8.

Your JavaScript may behave differently in the different document modes.
Here are three examples of code that, due to our conformance to the ES5
standard, provide different results depending upon the document mode. For
additional compatibility guidance and JavaScript feature changes in IE9, see
the
Compatibility Cookbook on MSDN, the blog post
Enhanced Scripting in IE9: ECMAScript 5 Support and More, the ES3
Standards Support document, and the
Microsoft JScript extensions to the ES3 standard.