What does this block of obfuscated JavaScript do?

I've seen a block of JavaScript code, that return an alert message with 1 on page load. Here is the code

($=[$=[]][(__=!$+$)[_=-~-~-~$]+({}+$)[_/_]+
($$=($_=!''+$)[_/_]+$_[+$])])()[__[_/_]+__
[_+~$]+$_[_]+$$](_/_)    

Can someone please explain what's going on here?