[javascript] Best way to check for IE less than 9 in JavaScript without library

You are all trying to overcomplicate such simple things. Just use a plain and simple JScript conditional comment. It is the fastest because it adds zero code to non-IE browsers for the detection, and it has compatibility dating back to versions of IE before HTML conditional comments were supported. In short,

var IE_version=(-1/*@cc_on,@_jscript_version@*/);

Beware of minifiers: most (if not all) will mistake the special conditional comment for a regular comment, and remove it

Basically, then above code sets the value of IE_version to the version of IE you are using, or -1 f you are not using IE. A live demonstration:

_x000D_
_x000D_
var IE_version=(-1/*@cc_on,@_jscript_version@*/);_x000D_
if (IE_version!==-1){_x000D_
    document.write("<h1>You are using Internet Explorer " + IE_version + "</h1>");_x000D_
} else {_x000D_
    document.write("<h1>You are not using a version of Internet Explorer less than 11</h1>");_x000D_
}
_x000D_
_x000D_
_x000D_

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