瀏覽器控制檯裏的$、$$是咋回事

程Hao同窗提到：
瀏覽器的控制檯裏，能夠直接使用$、$$方法，但是這些方法，卻不能被頁面裏的JS使用。

趙WenBo同窗給出解答：

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

谷歌的說明：

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.

點贊一下IE的某些版本，還能夠直接看到函數體：