Co

is_type.js

var is = {
        get_type: function(thing){
            if(thing===null) return "[object Null]";
            else if(thing!==thing) return "[object NaN]";
            return Object.prototype.toString.call(thing);
        },
        compare: function(type){
            return function(value){
                return this.get_type(value) === "[object " + type + "]";
            };
        }
    };
(function(is, types){
    types.map(function(type){is[type]=is.compare(type);});
}(is, ["Array","Object","Number","String","Undefined","Null","Boolean","NaN"]));

is.Array([1])       // true
is.Object({a:1})    // true
is.Number(1)        // true
is.String("test")   // true
is.String("")        // true
is.Undefined(undefined) // true
is.Null(null)      // true
is.Boolean(true)      // true
is.Boolean(false)     // true
is.Boolean(1)     // false
is.NaN(NaN)       // true