isArguments

isArguments.js

// Cross-browser but not absolutely strong.
function isArguments( obj ) {
	return typeof obj === "object" && ( "callee" in obj ) && typeof obj.length === "number";
}


// Ideally strong, but broken on Opera...
var isArguments = (function( undefined ) {

	var toStr = ({}).toString,
		
		returnTrue = function() {
			// To be sure it will not be inlined (future engines).
			return arguments !== undefined;
		},
		
		test = function( obj ) {
			return obj != null && toStr.call( obj ) === "[object Arguments]";
		};
		
	return test( arguments ) ? test :
		function( obj ) {
			// Using "in" works in strict mode too.
			if ( obj != null && "callee" in obj ) {
				try {
					return returnTrue.apply( this, obj );
				} catch (e) {}
			}
			return false;
		};
})();

Just note that your versions will fail with null or undefined values, so you should do obj != null as first check.

Yeah I forgot to put that in there on my initial post, but I added it in right afterwards. Doh!

https://gist.github.com/952521