Several things such as standard way for adding, removing and checking for keys, built-in size property, extensibility. You think "key in object" is better than

Several things such as standard way for adding, removing and checking for keys, built-in size property, extensibility. You think "key in object" is better than "maaaaap.has(key)"? I don't because it's unpredictable and prone to exceptions. Give them a try. It's not a complete replacement for regular old objects