浏览器控制台里的$、$$是咋回事

程Hao同学提到：
浏览器的控制台里，可以直接使用$、$$方法，可是这些方法，却不能被页面里的JS使用。

赵WenBo同学给出解答：

https://developers.google.com/web/tools/chrome-devtools/console/utilities   
https://developer.mozilla.org/en-US/docs/Tools/Web_Console/The_command_line_interpreter#Helper_commands

谷歌的说明：

The Console Utilities API contains a collection of convenience functions for performing common tasks: selecting and inspecting DOM elements, displaying data in readable format, stopping and starting the profiler, and monitoring DOM events.

Warning: These functions only work when you call them from the Chrome DevTools Console. They won't work if you try to call them in your scripts.

点赞一下IE的某些版本，还可以直接看到函数体：