How to type a line of JavaScript code in browser address bar to let Facebook Chat go offline?

When the Facebook web page is opened, I want to type a line of Ja开发者_运维知识库vaScript code in the browser address bar and let the Facebook Chat go offline. What JavaScript code should I use? And how can I analyze other JavaScript hacks for Facebook?

I tested some issues, and found that you cannot execute js from address bar, in fb opened page.