首先看一段ECMA中对Object.prototype.toString的解释：

Object.prototype.toString( )
When the toString method is called, the following steps are taken:

1. Get the [[Class]] property of this object.
2. Compute a string value by concatenating the three strings “[object “, Result (1), and “]”.
3. Return Result (2)

var typeOf = function() {
        var toString = Object.prototype.toString;
        var types = ['Object','Boolean', 
            'Number', 'String', 'Function', 
            'Array', 'Date', 'RegExp'];
        var map = {};

        for (var i = 0, l = types.length; i < l; i++) {
            var type = types[i];
            map['[object ' + type + ']'] = type.toLowerCase();
        }

        return function(item) {
            if (item === null) return 'null';
            if (item.nodeType) return 'node';
            if (item.length && item.callee) return 'arguments';
            if (typeof item === 'object') return map[toString.call(item)];
            return typeof item;
        };
    }();
window.typeOf = typeOf;