I'm reading es5 these days and find that [[configurable]] attribute in some built-in properties of global object is set to true which means we can delete these properties. For example: the join method of Array.prototype object have attributes {[[Writable]]:true, [[Enumerable]]: false, [[Configurable]]: true} So we can easily delete the join method for Array like: delete Array.prototype.join; alert([1,2,3].join); The alert will display undefined in my chromium 17,firefox 9 ,ie 10,even ie6; In Chrome 15 & safari 5.1.1 the [[configurable]] attribute is set to true and delete result is also true but the final result is still function(){[native code]}. Seems like this is a bug and chromium fix it. I haven't notice that before. In my opinion, delete built-in functions in user's code is dangerous, and will bring out so many bugs when working with others.So why ECMAScript make this decision?