Yahoo! UI Library

YAHOO.util.Lang = {
    isArray: function(val) { // frames lose type, so test constructor string
        if (val.constructor && val.constructor.toString().indexOf('Array') > -1) {
            return true;
        } else {
            return YAHOO.util.Lang.isObject(val) && val.constructor == Array;
        }
    },
        
    isBoolean: function(val) {
        return typeof val == 'boolean';
    },
    
    isFunction: function(val) {
        return typeof val == 'function';
    },
        
    isNull: function(val) {
        return val === null;
    },
        
    isNumber: function(val) {
        return !isNaN(val);
    },
        
    isObject: function(val) {
        return typeof val == 'object' || YAHOO.util.Lang.isFunction(val);
    },
        
    isString: function(val) {
        return typeof val == 'string';
    },
        
    isUndefined: function(val) {
        return typeof val == 'undefined';
    }
};