JavaScript: Can I detect IE9 if it's in IE7 or IE8 compatibility mode?

Solution 1:

Actually the user agent string is different for IE9 when being run in IE7 compatibility mode, so this would be one of the best ways to distinguish between different IE versions.

Introducing IE9’s User Agent String:

Similar to IE8, IE9’s Compatibility View will map to IE7 Standards Mode, and IE9’s UA string when in Compatibility View will be:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; Trident/5.0)
In Compatibility View, IE9 reports itself as IE7 through the application version number (Mozilla/4.0) and version token (MSIE 7.0). This is done for compatibility. An incremented Trident token, from ‘Trident/4.0’ to ‘Trident/5.0’, allows websites to distinguish between IE9 running in Compat View and IE8 running in Compat View.

(emphasis added by me). So the user agent string is the same as it reports itself being "Mozilla/4.0" and MSIE 7.0, but IE9 will always be Trident/5.0 - no matter whether it says MSIE 7.0, MSIE 8.0 or MSIE 9.0.

Actually you should check out this great compilation: Browser ID (User-Agent) Strings or even better useragentstrings.com

Solution 2:

document.documentMode is the best way for document mode.

Solution 3:

IE7 doesn't contain any information on Trident

User-Agent : Mozilla/4.0 (compatible; MSIE 7.0)

IE8 contains this string: "Trident/4.0"

User-Agent : Mozilla/4.0 (compatible; MSIE 8.0; Trident/4.0)

IE9 contains this string: "Trident/5.0"

IE9 in compatability mode:

User-Agent : Mozilla/4.0 (compatible; MSIE 7.0; Trident/5.0)

IE9 in normal mode:

User-Agent : Mozilla/5.0 (compatible; MSIE 9.0; Trident/5.0)

Solution 4:

Here is a table of all Browser and Document modes for IE9:

table of all Browser and Document modes

Solution 5:

I'm hoping that IE9 will have some property that exists and is testable regardless of whether it's in 7, 8 or 9 mode.

Check e.g. for style.opacity, it was introduced in IE9 and is available regardless of the compatibility-mode:

<![if IE]> 
<script>
if(typeof document.documentElement.style.opacity!='undefined')
{
  //this must be at least IE9 
}
</script>
<![endif]>