如何在HTML中的for循环中使用文档

I'm trying to write a "for" loop that will print out the properties associated with the "document" object (write, getElementById, etc.) on an HTML page. Not for anything practical, just for learning purposes. If use the "document" object directly, it works fine, but if I try to use a variable that has the value "document" it won't work. Here's the code I have:I'm trying to write a "for" loop that will prin