I'm a PHP guy and I wanna know how to do $_GET['something'] in vanilla JS

I'm making an imageboard and I basically want javascript to display the reply field when it detects that in the url there is &mode=reply
So I need to have in javascript something that does this
if isset $_GET['mode'] && mode == "reply" {
display div.postfield
]
I'd do everything in comfy PHP but it cannot open and close divs without a pageload.