isArguments

isArguments.js

var isArguments = (function( undefined ) {
    var toString = Object.prototype.toString,
        returnTrue = function() {
            return arguments !== undefined;
        };

    return function( obj ) {
        if ( obj != null ) {
            if ( toString.call( obj ) == "[object Arguments]" ) {
                return true;
            } else if ( "callee" in obj ) {
                try {
                    return returnTrue.apply( this, obj );
                } catch ( e ) {}
            }
        }

        return false;
    }
})();

The obj != null check have to be even before toSting.call.

Btw. seams cls is superfluous in your case.

Hmm, I thought so too but it seems to work in the else if -- good call on the cls. null/undefined seem to return [object Window].

You are right about null/undefined returning [object Window], but note that in strict mode it will throw an exception.

Yeah, I should be strict mode compliant. Also I was happy to see [object Null] and [object Undefined] in FF 4.0, that could be helpful.

Yea, seams I was wrong on that. Hope other browsers are same...